Real Time Interview Question

Some dba interview question generally faced by me. 1)How do you verify the No. of Databases running on a Host A: ps -ef|grep pmon either check cat /etc/oratab/ file 2)How do you verify the name of the database A:select name from v$database; Or Show parameter db_name; 3)How do you verify whether your database is running o rnot A: Here is how to check if your database is running: check_stat=`ps -ef|grep ${ORACLE_SID}|grep pmon|wc -l`; oracle_num=`expr $check_stat` if [ $oracle_num -lt 1 ] then exit 0 fi 4)How do you verify when the database is created 5)How do you verify since when your database is running 6)How do you verify the name of your instance A:Sql>ssshow instance; 7)How do you verify the mode of your database A: SQL> SELECT LOG_MODE FROM SYS.V$DATABASE; LOG_MODE -----------NOARCHIVELOG SQL> startup mount SQL> alter database archivelog; SQL> alter database open;

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There are several system views that can provide us with information reguarding archives, such as: V$DATABASE Identifies whether the database is in ARCHIVELOG or NOARCHIVELOG mode and whether MANUAL (archiving mode) has been specified. V$ARCHIVED_LOG Displays historical archived log information from the control file. If you use a recovery catalog, the RC_ARCHIVED_LOG view contains similar information. V$ARCHIVE_DEST Describes the current instance, all archive destinations, and the current value, mode, and status of these destinations. V$ARCHIVE_PROCESSES Displays information about the state of the various archive processes for an instance. V$BACKUP_REDOLOG Contains information about any backups of archived logs. If you use a recovery catalog, the RC_BACKUP_REDOLOG contains similar information. V$LOG Displays all redo log groups for the database and indicates which need to be archived. V$LOG_HISTORY Contains log history information such as which logs have been archived and the SCN range for each archived log. Using these tables we can verify that we are infact in ARCHIVELOG mode: SQL> select log_mode from v$database; LOG_MODE a

-----------ARCHIVELOG SQL> select DEST_NAME,STATUS,DESTINATION from V$ARCHIVE_DEST 8)How do you enable automatic archiving A: SQL> shutdown SQL> startup mount SQL> alter database archivelog; SQL> alter database open; 9)How do you do manual archiving A: SQL>startup mount SQL>alter database archivelog manual; SQL> archive log list 10)How do you set the archive file format 11)What is the physical structure of your database 12)How do you verify whether instance is using pfile or spfile A: 1) SELECT name, value FROM v$parameter WHERE name = 'spfile'; //This query will return NULL if you are using PFILE 2) SHOW PARAMETER spfile // This query will returns NULL in the value column if you are using pfile and not spfile 3) SELECT COUNT(*) FROM v$spparameter WHERE value IS NOT NULL; // if the count is non-zero then the instance is using a spfile, and if the count is zero then it is using a pfile: By Default oracle will look into the default location depends on the o/s. Like in unix, oracle will check in $oracle_home/dbs directory and on windows it will check in oracle_home/database directory, and the content of pfile is just text based, but spfile content is in binary format, that is understandable by oracle very well. 13)How do you start an instance with spfile A: SQL> startup pfile=\\C0027\database\SPFILEZWEITEDB.ORA 14)How do you start an instance with pfile 15)How do you read the contents of spfilesss & A: i could read the content of the spfile with , Vi editor 16)How do you change the contents of spfile a

A: it's content can ,only be altered with SQL>alter system set scope=memory or spfile;

17)List out the Initialisation parameters used by your instance A: Parameter Name

Description

BACKGROUND_DUMP_DEST:Specifies the directory where the trace files generated by the background processes are to be written. This is also the location of the alert log for the database. COMPATIBLE: Provides Oracle with the understanding of what features you intend the database to have. If there is a feature in 9i that was not available in 8i and this parameter is set to 8.1.7, the feature will fail to perform. CONTROL_FILES :The location of the control files for the database. DB_BLOCK_SIZE:The default block size for the database. USER_DUMP_DEST:Specifies the directory where the trace files generated by user sessions are written.

CORE_DUMP_DEST :Specifies the location where core dump files generated by Oracle are written. DB_NAME :The name of the database and also of the SID. INSTANCE_NAME :The name of the instance and, with the exception of a RAC environment, also the database and the SID.

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OPEN_CURSORS :The maximum number of cursors that you want to have opened in your instance at any given time.

18)Who is the owner of your oracle software 19)What is the version of your database A: SQL> select * from v$version; 20)What is the version of your sqlplus 21)Where is the SQLPLUs located A: $ORACLE_HOME/bin directory 22)Who is the owner of data dictionary A: The SYS user owns the data dictionary. The SYS and SYSTEM users are created when the database is created. 23)Where is data dictionary located 24)What are the dynamic views available in Nomount stage 25)What are the dynamic views available in Mount stage 26)What are the data dictionary views available in NOmount Stage 27)What are the data dictionary view available in Mount STage 28)How do you change the database from a Mount stage to Nomount stage 29)How do you view the current log sequence No. A:sql> ARCHIVE LOG LIST; Below it will display current sequence number 30)What is the difference between instance name and databasename A: Database: A collection of physical operating system files or disk. When using Oracle 10g Automatic Storage Management (ASM) or RAW partitions, the database may not appear as individual separate files in the operating system, but the definition remains the same.

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* Instance: A set of Oracle background processes/threads and a shared memory area, which is memory that is shared across those threads/processes running on a single computer 31)Write down the steps to change the database mode to NoarchiveLog A: SQL> select log_mode from v$database; LOG_MODE -----------NOARCHIVELOG SQL> startup nomount; SQL> alter database noarchivelog; SQL> alter database open; 32)What are the contents of the alert log file 33)Where are the Background processes logging information written to 34)How do you specify the location of the Background processes logging information 35)How do you specify the location of the User Processes logging information Installing Oracle9i/10g at unix (RedHat) environment 2) What are the components of physical database structure of Oracle database 3) What is the difference between materialized view and snapshots A: Snapshot is a copy of a table on a remote system but materialized view is used to make a snapshot of a table available on a remote system. 4) What is difference between base table and data dictionary views? A: Base tables are made when the database is created and these tables are stored in data dictionary. these base tables contains the information related to the database. these tables are own by sys. the information in these tables is crypted and cannot be modified. So we use the views to access the information in these base tables. these views are called data dictionary views. these views are created when we run the script a

@ ORACLE_HOME/RDBMS/ADMIN/CATALOG.SQL 5) Cloning and Standby Databases A: Cloning is nothing but the copy of your database which can be open in read write mode. The standby database is also a copy of your database which is in standby mode and which is made in sink with production database by applying the redo log generated at source database (prodcution database). This database can not be open in read write mode. This standby database can be mode in read write mode by activating the database which reset's it's redo log sequence. 6) What is SCN number in Oracle? Plz any one give me the explanation for SCN A: The system change number (SCN) is an ever-increasing value that uniquely identifies a committed version of the database. Every time a user commits a transaction Oracle records a new SCN. You can obtain SCNs in a number of ways for example from the alert log. You can then use the SCN as an identifier for purposes of recovery. 7) Is VARCHAR2 size optimization worthwhile ? 8) How to manager Memory in Oracle Database? How to maximize nos. of user in Oracle Database? 9) Index tablespace for a Database A: There is no such provision in oracle to have default index tablespace. Workaround is- you can have a job which will scan for indexes in other tablespaces and rebuild into desired one. 10) What are the fixed memory structures inside the SGA? A: Part of the SGA contains general information about the state of the database and the instance which the background processes need to access; this is called the fixed SGA. No user data is stored here. The SGA also includes information communicated between processes such as locking information. With the dynamic SGA infrastructure the size of the buffer cache the shared pool the large pool and the process-private memory can be changed without shutting down the instance. Dynamic SGA allows Oracle to set at run time limits on how much virtual memory Oracle uses for the SGA. Oracle can start instances underconfigured and allow the instance to use more memory by growing the SGA components up to a maximum of SGA_MAX_SIZE.

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11) what is directory naming in oracle9i ? A: Oracle Net Services use a centralized directory server as one of the primary methods for storage of connect identifiers. Clients configured directory usage can use the connect identifiers in their connect string. The directory server resolves the connect identifier to a connect descriptor that is passed back to the client. oracle net services support oracle internet directory and microsoft active directory 12) What is the most important action a DBA must perform after changing the database from NOARCHIVELOG TO ARCHIVELOG ? A: backup the entire database...b’coz, alter system archive log start otherwise the database halts if unable to rotate redo logs 13) What is the difference between Pctused and PctFree? A: PCT USERD - The Percentage of Minimum space user for insertion of New Row data. The value determines when the block gets into the FREELISTS structure PCTFREE - The Percentage of Space reserved for future updation of existing data 14) how to find which tablespace belongs to which datafile ? A: SQL> select tablespace_name,file_name from dba_data_files; 15) What is a synonym A: A synonym is an alternative name for objects such as tables, views, sequences, stored procedures, and other database objects. SQL> CREATE SYNONYM emp FOR SCOTT.EMP; SQL> DROP SYNONYM emp 16) What is a Schema ? 17) What is a deadlock ? Explain . A: Two processes wating to update the rows of a table which are locked by the other process then deadlock arises. 18) What is a latch? A: A latch is a serialization mechanism. In order to gain access to a shared data structure, you must "latch" that structure. that will prevent others from modifying it while you are looking at it or modifying it your self. It is a programming tool. a

19) Latches vs Enqueues A: Enqueues are another type of locking mechanism used in Oracle. An enqueue is a more sophisticated mechanism which permits several concurrent processes to have varying degree of sharing of "known" resources. Any object which can be concurrently used, can be protected with enqueues. A good example is of locks on tables. We allow varying levels of sharing on tables e.g. two processes can lock a table in share mode or in share update mode 20) What is difference between Logical Standby Database and Physical Standby database? A: Physical standby differs from logical standby: Physical standby schema matches exactly the source database. Archived redo logs and FTP'ed directly to the standby database which is always running in "recover" mode. Upon arrival, the archived redo logs are applied directly to the standby database. Logical standby is different from physical standby:Logical standby database does not have to match the schema structure of the source database. Logical standby uses LogMiner techniques to transform the archived redo logs into native DML statements (insert, update, delete). This DML is transported and applied to the standby database. Logical standby tables can be open for SQL queries (read only), and all other standby tables can be open for updates.Logical standby database can have additional materialized views and indexes added for faster performance. 21) Explain about Oracle Statistics parameter in export? A: Export is one of taking backup. In export backup you can specify many parameters in par file or at command line. One of the parameter is statistics=y or n. If you specify y it is going to export statistics generated in the database placed in dump file.22) what is difference between latch,locks and enqueue ? 23) Waht is the frequency of log Updated..? A: 1.COMMIT or ROLLABCK 2.time out occurs (3 secs) 3 1/3 of log is full 4. 1 mb of redo 5. Checkpoint occurs a

LGWR writes: 1) COMMIT/ ROLLBACK 2) 1 MB of large transaction 3) Before DBWR writes 4) 1/3 of REDO LOG is full 5) time out occurs 6) Check Point encounters whenever commit,checkpoint or redolog buffer is 1/3rd full 24) Which process writes data from data files to database buffer cache? 25) what is the difference between local managed tablespace & dictionary managed tablespace ? A: The basic diff between a locally managed tablespace and a dictionary managed tablespace is that in the dictionary managed tablespace every time a extent is allocated or deallocated data dictionary is updated which increases the load on data dictionary while in case of locally managed tablespace the space information is kept inside the datafile in the form of bitmaps every time a extent is allocated or deallocated only the bitmap is updated which removes burden from data dictionary. 26) What is clusters ? A: A cluster is a data structure that improves retrieval performance. A cluster, like an index, does not affect the logical view of the table. A cluster is a way of storing related data values together on disk. Oracle reads data a block at a time, so storing related values together reduces the number of I/O operations needed to retrieve related values, since a single data block will contain only related rows. A cluster is composed of one or more tables. The cluster includes a cluster index, which stores all the values for the corresponding cluster key. Each value in the cluster index points to a data block that contains only rows with the same value for the cluster key. 27) What is an extent 28) Database Auto extend question A: This is an Interview Question By BMC Software.. " while installing the Oracle 9i ( 9.2) version, automatically system takes the space of approximately 4 GB.. thats fine.... Now, if my database is growing up and it is reaching the 4GB of my database space...Now, i would like to a

extend my Database space to 20 GB or 25 GB... what are the things i have to do " Pls give me the accurate Solutions or alternates for this Query. 29) How to know which query is taking long time? 30) where does the SCN resides (system change number) A: SCN changes for every 3 minutes in 10g and for each and every action in 9i. It resides in control files and data files. CKPT (checkpoint) background process updates the SCN numbers from control files to the datafiles. SMON (system monitor) background process checks for the SCN numbers to be same in both datafiles and control files while starting the database. If same then only the database is consistent. Otherwise the database will not start. 31) What is RAC? What is Data Migration? What is Data Pumping? 32) Is it possible to drop more than one table using single sql statement? if yes then how. A: No because we can drop only one table or table data by using drop. 33) One DDL SQL script that has kept at certain location should be run on multiple servers to keep database synchronize. This task has to do in oracle database and this should be done as a job from scheduler. How will you do it? A: There are many ways to do that. Following is the one of the ways I would prefer as I do it usually. You can achieve this by having a small Unix Scripting / Windows Shell Scripting / any other scripting (including PR*C). This Unix/WSH/ script has to go through a loop for each database and get connect and execute the SQL script. 34) How to you move from Dedicated server Process to a Shared Server Process A: Use DBCA toolYou will get the option to select shared server mode. 1. set SHARED_SERVERS=(more than 1) in init.ora 2. make changes in tnsnames.ora file to get the connection with DISPATHERS rather than dedicated servers 35) What are the attributes of the Virtual Indexes A: It does not store any data value in it unlike normal index do.Queries will not get benefitted. This can be used only for analysis. 1. These are permanent and continue to exist unless we drop them. 2. Their creation will not affect existing and new sessions. Only a

sessions marked for Virtual Index usage will become aware of their existence. 3. Such indexes will be used only when the hidden parameter _use_nosegment_indexes is set to true. 36) HOW 2 ENABLE PARTITIONING FEAUTURE IN ORACLE 8i 37) You have taken import of a table in a database. you have got the Integrity constraint violation error. How you are going to resolve it. A: use this DDL statement in create table script to avoid integrity constraint violation error DROP TABLE tabl_name CASCADE CONSTRAINTS ; cascade constraints delete foreign keys associated with table and table frees with foreign keys. 38) Why in 10G, when you use real time apply feature in conjunction with Maximum Protection, you can achive zero data loss, but not zero database downtime?? A: When you say If the last standby database configured in this mode becomes unavailable processing stops on the primary database. does 'processing stops' mean committing a transaction stops on the primary database...if so there is data loss correct? 39) What is ORA-1555? A: ORA-1555 error can occur in Oracle 10g also even with UNDO RETENTION GUARANTEE enabled. ORA-1555 happens when Oracle server process could not find the block-image in UNDO tablespace for the read-consistency. 40) How can the problem be resolved if a SYSDBA, forgets his password for logging into enterprise manager? A: I think there are 2 ways to do that. 1. Login as SYSTEM and change the SYS password by using ALTER USER. 2. Recreate the password file using orapwd and set remote_password_file=exclusive and then restart the instance. 41) What is the correct sequence among FETCH, EXECUTE, And PARSE A: 1. Parse 2. Execute 3. Fetch 42) What is database link A: A database link is a pointer in the local database that allows you to access on a remote database. a

43) What is a Database instance ? Explain 44) What is an Index ? How it is implemented in Oracle Database ? 45) What is Parallel Server ? 46) What is a deadlock and Explain 47) What are the components of logical database structure of Oracle database 48) What is an Oracle index 49) What is a tablespace 50) When a database is started, Which file is accessed first? 51) How to handle data curreption for ASM type files? 52) Do a view contain data A: No, View never contain the the data, Only defination of it stores in the data base, when ever you invoke them they show you the data based on their defination.Only Materlized view or SnaptShot contain the the data. 53) How many maximum number of columns can be part of Primary Key in a table in Oracle 9i and 10g? A: The maximum number of columns that can be a part of Primary key in a table in Oracle 10g is 32. 54) I am getting error "No Communication channel" after changing the domain name? what is the solution? A: Change the domain name in the sqlnet.ora file in NAMES.DEFAULT_DOMAIN parameter. 55) When a user comes to you and asks that a particular SQL query is taking more time. How will you solve this? A: If you find the Sql Query (which make problem) then take a Sqltrace with explain plan it will show how the sql query will executed by oracle depending upon the report you will tune your database

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for example: one table have 10000 record but you want to fetch only 5 rows but in that query oracle does the full table scan. only for 5 rows full table is scan is not a good thing so create a index on the particular column by this way to tune the datatabse 56) How to find how many database reside in Oracle server in query? A: select count(*) from v$database; Or open oratab 57) What process writes from data files to buffer cache? 58) Can you tell something about Oracle password Security? A: If user authentication is managed by the database security administrators should develop a password security policy to maintain database access security. For example database users should be required to change their passwords at regular intervals and of course when their passwords are revealed to others. By forcing a user to modify passwords in such situations unauthorized database access can be reduced. Set the ORA_ENCRYPT_LOGIN environment variable to TRUE on the client machine. Set the DBLINK_ENCRYPT_LOGIN server initialization parameter to TRUE. 59) What is the function of redo log A: redo log is a part of physical structure of oracle. its basic function is to record all the changesmade to daatabase information. wheneveer an abnormal shutdown take place preventing system to update the database changes can be obtained from redolog and hence the changes are not lost. 60) What is SYSTEM tablespace and when is it created 61) How to DROP an Oracle Database? A: You can do it at the OS level or go to dbca and click on delete database 62) what is RAP? 63) Can you start a database without SPfile in oracle 9i? A:no a

64) Where we use bitmap index ? A: Bitmap indexes are most appropriate for columns having low distinct values 65) what are the diffrent file types that are supported by SQL*Loader? A: 1. .txt 2. .dat 3. .csv 4. .mdb 66) how do sql statement processing oracle database? A: When a select statement is executed first of all the statements hash code is genrated then that hash code is matched in library cache if the hash code matched then statement is directly executed and if the hash code is not present then hard parsing is done and statement is executed 67) How to Estimate the size of Tablespace??? 68) How to query to know the structure of a single Database and from more than one database. 69) how to estimate size of database? 70) What is difference between spfile and init.ora file??? 72. Explain the relationship among database, tablespace and data file.What is schema A: -- A Oracle Database consists of one or more tablespaces --- Each Table space in an Oracle database consists of one or more files called datafiles. --- A database's data is collectively stored in the datafiles that constitute each tablespace of the database. 73. Name init.ora parameters which effects system performance. A: These are the Parameters for init.ora DB_BLOCK_BUFFERS SHARED_POOL_SIZE SORT_AREA_SIZE

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DBWR_IO_SLAVES ROLLBACK_SEGMENTS SORT_AREA_RETAINED_SIZE DB_BLOCK_LRU_EXTENDED_STATISTICS SHARED_POOL_RESERVE_SIZE

74. What is public database link A: Database link is a schema object in one database to access objects in another database. When you create database link with Public clause it is available for access to all the users and if you omit this clause then database link is privat and available only to you. 75. What are the uses of rollback segment A: The uses of Roll Back Segment are : 1. Transaction Rollback 2. Transaction Recovery 3. Read Consistency 76. What is the use of control file 77. What is difference between SQLNET.ORA AND TNSNAMES.ORA AND LISTENER.ORA?? A: Oracle uses all three files (tnsnames.ora, sqlnet.ora, listener.ora) for network configuration. 78. What is the difference between .ora and net file or .ora and .net or tnsnames.ora sqlnet.ora listener.ora what ever the differnence makes between ora and net. A: .ora files contain Oracle Engine papameters info .net files contain O.S engine parameter info 79. What are materialized views? when are they used? A: Use of Meterialized view:Expensive operations such as joins and aggregations do not need to be reexecuted. If the query is astisfied with data in a Meterialized view, the server transforms the query to reference the view rather than the base tables. 80. How many memory layers are in the shared pool? a

81. What is the database holding Capacity of Oracle ? A: database holding capacity of oracle 10 g is 8 trillion tera bytes. 82. How do you rename a database? A: STEP 1: Backup the database. STEP 2: Mount the database after a clean shutdown: SHUTDOWN IMMEDIATE STARTUP MOUNT STEP 3: Invoke the DBNEWID utility (nid) specifying the new DBNAME from the command line using a user with SYSDBA privilege: nid TARGET=sys/password@TSH1 DBNAME=TSH2 Assuming the validation is successful the utility prompts for confirmation before performing the actions. Typical output may look something like: C:\oracle\920\bin>nid TARGET=sys/password@TSH1 DBNAME=TSH2 DBNEWID: Release 9.2.0.3.0 - Production Copyright (c) 1995, 2002, Oracle Corporation. All rights reserved. Connected to database TSH1 (DBID=1024166118) Control Files in database:

C:\ORACLE\ORADATA\TSH1\CONTROL01.CTL

C:\ORACLE\ORADATA\TSH1\CONTROL02.CTL C:\ORACLE\ORADATA\TSH1\CONTROL03.CTL Change database ID and database name TSH1 to TSH2? (Y/[N]) => Y Proceeding with operation Changing database ID from 1024166118 to 1317278975 Changing database name from TSH1 to TSH2 Control File C:\ORACLE\ORADATA\TSH1\CONTROL01.CTL - modified Control File C:\ORACLE\ORADATA\TSH1\CONTROL02.CTL - modified a

Control File C:\ORACLE\ORADATA\TSH1\CONTROL03.CTL - modified Datafile C:\ORACLE\ORADATA\TSH1\SYSTEM01.DBF - dbid changed, wrote new name Datafile C:\ORACLE\ORADATA\TSH1\UNDOTBS01.DBF - dbid changed, wrote new name Datafile C:\ORACLE\ORADATA\TSH1\CWMLITE01.DBF - dbid changed, wrote new name Control File C:\ORACLE\ORADATA\TSH1\CONTROL01.CTL - dbid changed, wrote new name Control File C:\ORACLE\ORADATA\TSH1\CONTROL02.CTL - dbid changed, wrote new name Control File C:\ORACLE\ORADATA\TSH1\CONTROL03.CTL - dbid changed, wrote new name Database name changed to TSH2. Modify parameter file and generate a new password file before restarting. Database ID for database TSH2 changed to 1317278975. All previous backups and archived redo logs for this database are unusable. Shut down database and open with RESETLOGS option. Succesfully changed database name and ID. DBNEWID - Completed succesfully. STEP 4: Shutdown the database: SHUTDOWN IMMEDIATE STEP 5: Modify the DB_NAME parameter in the initialization parameter file. The startup will result in an error but proceed anyway. STARTUP MOUNT ALTER SYSTEM SET DB_NAME=TSH2 SCOPE=SPFILE; a

SHUTDOWN IMMEDIATE STEP 6: Create a new password file: orapwd file=c:\oracle\920\database\pwdTSH2.ora password=password entries=10 STEP 7: Rename the SPFILE to match the new DBNAME. STEP 8: If you are using Windows you must recreate the service so the correct name and parameter file are used: oradim -delete -sid TSH1 oradim -new -sid TSH2 -intpwd password -startmode a -pfile c:\oracle\920\database\spfileTSH2.ora If you are using UNIX/Linux simply reset the ORACLE_SID environment variable: ORACLE_SID=TSH2; export ORACLE_SID STEP 9: Alter the listener.ora and tnsnames.ora setting to match the new database name and restart the listener: lsnrctl reload STEP 10: Open the database with RESETLOGS: STARTUP MOUNT ALTER DATABASE OPEN RESETLOGS; STEP 11: Backup the database. 83. What are clusters 84. What is private database link 85. How can be determine the size of the database? A: select sum(bytes)/1024/1024/1024 Size_in_GB from dba_data_files 86. Can you name few DBMS packages and their use? A: DBMS_METADATA DBMS_STATS a

DBMS_SUPPORT DBMS_SESSION 87. What is the view name where i can get the space in MB for tables or views? 88. Assuming today is Monday, how would you use the DBMS_JOB package to schedule the execution of a given procedure owned by SCOTT to start Wednesday at 9AM and to run subsequently every other day at 2AM? 89. How can you check which user has which Role. A: select * from dba_role_privs order by grantee; 90. How do you find wheather the instance was started with pfile or spfile 91. What are the Advantages of Using DBCA A: You can use its wizards to guide you through a selection of options providing an easy means of creating and tailoring your database. It allows you to provide varying levels of detail. You can provide a minimum of input and allow Oracle to make decisions for you, eliminating the need to spend time deciding how best to set parameters or structure the database. Optionally, it allows you to be very specific about parameter settings and file allocations. 92. State new features of Oracle 10g. 93. What spfile/init.ora file parameter exists to force the CBO to make the execution path of a given statement use an index, even if the index scan may appear to be calculated as more costly? A: CBO (Cost Based Optimizer):Generates an execution plan for a SQL statement optimizer_index_cost_adj parameter can be set to help CBO to decide an execution plan which effects the speed of SQL query. we can also make necessary changes to the following parameters to effect CBO performance: optimizer_search_limit & optimizer_max_permutations 94. What is a redo log a

A: The Primary function of the redo log is to record all changes made to data. 95. can we create index on long raw column? A: NO we can't create index on long raw column. 96. What does database do during mounting process? A: During database mount process, Oracle would check for the existence of controlfiles mentioned in init.ora file but it wont check the contents of the controlfile which is done during the opening of database. 97. What is a database instance and Explain 98. What is Oracle table 99. What are the characteristics of data files A: A data file can be associated with only one database. Once created a data file can't change size.One or more data files form a logical unit of database storage called a tablespace. 71) What are the different types of segments 72) What are the Advantages of Using DBCA 84) What are the types of database links A: Private Database Link: You can create a private database link in a specific schema of a database. Only the owner of a private database link or PL/SQL subprograms in the schema can use a private database link to access data and database objects in the corresponding remote database. Public Database Link : You can create a public database link for a database. All users and PL/SQL subprograms in the database can use a public database link to access data and database objects in the corresponding remote database. Global Database Link - When an Oracle network uses Oracle Names the names servers in the system automatically create and manage global database links for every Oracle database in the network. All users and PL/SQL subprograms in any database can use a global database link to access data and database objects in the corresponding remote database. 85) When can hash cluster used A: Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is a

hashed. The resulting hash key value points directly to the area on disk that stores the specified rows. 86) What is cluster key A: The related columns of the tables in a cluster is called the Cluster Key. 87) What is a private synonym 88) What is an Oracle view 89) What are Schema Objects A: Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links. 90) Can a tablespace hold objects from different schemes A: It can be the only required option is that your tablespace have quota assigned to any user that want to store objects in it. 91) What is a segment 92) What is row chaining A: In Circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs , the data for the row is stored in a chain of data block (one or more) reserved for that segment. 93) What is an index and How it is implemented in Oracle database A: Indexes are used both to improve performence and to ensure uniquness of a column. Oracle automatically creates an index when a UNIQUE or PRIMARY key constarints clause is specified in a create table command. 94) What is a schema 95) What does a control file contains 96) How to define data block size A: stansard block size which is set with parameter DB_BLOCK_SIZE cannot be changed after creating database. We can set non standard parameter size later with parameter DB_nk_BLOCK_SIZE and it can be changed. 97) What is hash cluster

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98) What is index cluster 99) What are clusters 100) How are the index updates 101) What is a public synonym 102) What is an Oracle sequence 103) Can a view based on another view 104) What is the use of redo log information A: The Information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database's data files. 105) How do you pin an object. 106) Is it possible to configure primary server and stand by server on different OS? A: NO. Standby database must be on same version of database and same version of Operating system. 107) Explain Oracle memory structure.

Oracle uses memory to store information such as the following:     

Program code Information about a connected session, even if it is not currently active Information needed during program execution (for example, the current state of a query from which rows are being fetched) Information that is shared and communicated among Oracle processes (for example, locking information) Cached data that is also permanently stored on peripheral memory (for example, data blocks and redo log entries)

The basic memory structures associated with Oracle include: 

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System Global Area (SGA), which is shared by all server and background processes and holds the following: o Database buffer cache o Redo log buffer o Shared pool o Large pool (if configured)

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Program Global Areas (PGA), which is private to each server and background process; there is one PGA for each process. The PGA holds the following: o Stack areas o Data areas

Figure 7-1 illustrates the relationships among these memory structures. Figure 7-1 Oracle Memory Structures

108) What are memory structures in Oracle? 109) What is a datafile 110) What is data block 111) What are synonyms used for A: Synonyms are used to : Mask the real name and owner of an object. 112) What is a cluster Key ? 113) What are the basic element of Base configuration of an oracle Database ? A: It consists of one or more data files. one or more control files.

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two or more redo log files. The Database contains multiple users/schemas one or more rollback segments one or more tablespaces Data dictionary tables User objects (table,indexes,views etc.,) The server that access the database consists of SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool) SMON (System MONito) PMON (Process MONitor) LGWR (LoG Write) DBWR (Data Base Write) ARCH (ARCHiver) CKPT (Check Point) RECO Dispatcher User Process with associated PGS 114) How Materialized Views Work with Object Types and Collections

11.Backup and Recovery Interview Questions Some of the Common Backup and Recovery Interview Questions for Oracle Database Administrator. These questions are common for both Senior Oracle DBA or Junior DBA. I have compiled these questions

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based upon the feedback I got from many candidates who have attended interviews in various MNC's 1. Which types of backups you can take in Oracle? 2. A database is running in NOARCHIVELOG mode then which type of backups you can take? A: If your Databse is in No Archivelog Mode then you must take a Cold backup of your Database. 3. Can you take partial backups if the Database is running in NOARCHIVELOG mode? 4. Can you take Online Backups if the the database is running in NOARCHIVELOG mode? A:no 5. How do you bring the database in ARCHIVELOG mode from NOARCHIVELOG mode? 6. You cannot shutdown the database for even some minutes, then in which mode you should run the database? 7. Where should you place Archive logfiles, in the same disk where DB is or another disk? 8. Can you take online backup of a Control file if yes, how? 9. What is a Logical Backup? 10. Should you take the backup of Logfiles if the database is running in ARCHIVELOG mode? 11. Why do you take tablespaces in Backup mode? 12. What is the advantage of RMAN utility? Advantage over tradition backup system: 1). copies only the filled blocks i.e. even if 1000 blocks is allocated to datafile but 500 are filled with data then RMAN will only create a backup for that 500 filled blocks. 2). incremental and accumulative backup. 3). catalog and no catalog option.

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4). detection of corrupted blocks during backup; 5). can create and store the backup and recover scripts. 6). increase performance through automatic parallelization( allocating channels), less redo generation.

What is Channel? Latest Answer : Channel is a link that RMAN requires to link to target database. This link is required when backup and recovery operations are performed and recorded. This channel can be allocated manually or can be preconfigured by using automatic channel ...

13. How RMAN improves backup time? A: Add channel to improve the performance of rman but it create session on DB and I/O on disk will increase so configure channel at proper number.

14. Can you take Offline backups using RMAN?

Recall that an offline backup is a backup of the database while it is not running. Hence, to perform our backup we will shutdown the database from RMAN and then mount the database. We will perform the backup. Once the backup is complete we will restart the database again. Here is an example of this process: RMAN>shutdown immediate RMAN>startup mount RMAN>backup database; RMAN>sql ’alter database open’;

Once this process is complete, you have completed your first backup

15. How do you see information about backups in RMAN? A: RMAN> List Backup; 16. What is a Recovery Catalog?

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A recovery catalog can be used to store metadata about multiple target databases. The tables and views constituting a recovery catalog are owned by a recovery catalog schema. Oracle recommends creating a recovery catalog schema in a separate dedicated database and not in the target database. A database containing a recovery catalog schema is called a recovery catalog database.

A: Recovery catalog is a repository of metadata that is available in the control file of the target database. Whenver we take backups using RMAN the copy of the backup is placed in the control file in the form of reusable records and as well as in the recovery catalog in the form of tables. So that while taking recovery also these table info is useful to apply the backup data 17. Should you place Recovery Catalog in the Same DB? a: Recovery catalog not in same target db Can take backup without catalog 18. Can you use RMAN without Recovery catalog? 19. Can you take Image Backups using RMAN? 20. Can you use Backupsets created by RMAN with any other utility? 20.what is difference b/w hot backup & Rman backup?

To take both backups we should keep database in archive log mode. RMAN will take the backup of database used block only where as hot backup will take physical existing database files completely

21. Where RMAN keeps information of backups if you are using RMAN without Catalog? A: RMAN keeps information of backups in the control file.

22. You have taken a manual backup of a datafile using o/s. How RMAN will know about it? 23. You want to retain only last 3 backups of datafiles. How do you go

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for it in RMAN? 24. Which is more efficient Incremental Backups using RMAN or Incremental Export? 25. Can you start and shutdown DB using RMAN? 26. How do you recover from the loss of datafile if the DB is running in NOARCHIVELOG mode? 27. You loss one datafile and it does not contain important objects. The important objects are there in other datafiles which are intact. How do you proceed in this situation? 28. You lost some datafiles and you don't have any full backup and the database was running in NOARCHIVELOG mode. What you can do now? 29. How do you recover from the loss of datafile if the DB is running in ARCHIVELOG mode? 30. You loss one datafile and DB is running in ARCHIVELOG mode. You have full database backup of 1 week old and partial backup of this datafile which is just 1 day old. From which backup should you restore this file? 31. You loss controlfile how do you recover from this? c 32. The current logfile gets damaged. What you can do now? 33. What is a Complete Recovery? 34. What is Cancel Based, Time based and Change Based Recovery? 35. Some user has accidentally dropped one table and you realize this after two days. Can you recover this table if the DB is running in ARCHIVELOG mode? 36. Do you have to restore Datafiles manually from backups if you are doing recovery using RMAN? 37. A database is running in ARCHIVELOG mode since last one month. A datafile is added to the database last week. Many objects are created in this datafile. After one week this datafile gets damaged before you can take any backup. Now can you recover this datafile when you don't have any backups?

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38. How do you recover from the loss of a controlfile if you have backup of controlfile? 39. Only some blocks are damaged in a datafile. Can you just recover these blocks if you are using RMAN? 40. Some datafiles were there on a secondary disk and that disk has become damaged and it will take some days to get a new disk. How will you recover from this situation? 41. Have you faced any emergency situation. Tell us how you resolved it? 42. At one time you lost parameter file accidentally and you don't have any backup. How you will recreate a new parameter file with the parameters set to previous values. some more oracle dba interview questions 1. explain the difference between a hot backup and a cold backup and the benefits associated with each. A:a hot backup is basically taking a backup of the database while it is still up and running and it must be in archive log mode. a cold backup is taking a backup of the database while it is shut down and does not require being in archive log mode. the benefit of taking a hot backup is that the database is still available for use while the backup is occurring and you can recover the database to any ball in time. the benefit of taking a cold backup is that it is typically easier to administer the backup and recovery process. in addition, since you are taking cold backups the database does not require being in archive log mode and thus there will be a slight performance gain as the database is not cutting archive logs to disk. 2. you have just had to restore from backup and do not have any control files. how would you go about bringing up this database? A:i would create a text based backup control file, stipulating where on disk all the data files where and then issue the recover command with the using backup control file clause. 3. how do you switch from an init.ora file to a spfile? a

A:issue the create spfile from pfile command. 4. explain the difference between a data block, an extent and a segment. A:a data block is the smallest unit of logical storage for a database object. as objects grow they take chunks of additional storage that are composed of contiguous data blocks. these groupings of contiguous data blocks are called extents. all the extents that an object takes when grouped together are considered the segment of the database object. 5. give two examples of how you might determine the structure of the table dept. A:use the describe command or use the dbms_metadata.get_ddl package. 6. where would you look for errors from the database engine? A:in the alert log. 7. compare and contrast truncate and delete for a table.A:both the truncate and delete command have the desired outcome of getting rid of allthe rows in a table. the difference between the two is that the truncate command is a ddl operation and just moves the high water mark and produces anow rollback. the delete command, on the other hand, is a dml operation, which will produce a rollback and thus take longer to complete. 8. give the reasoning behind using an index. A:faster access to data blocks in a table. 9. give the two types of tables involved in producing a star schema and the type of data they hold. A:fact tables and dimension tables. a fact table contains measurements while dimension tables will contain data that will help describe the fact tables. a

10. what type of index should you use on a fact table? A:a bitmap index. 11. give two examples of referential integrity constraints. A:a primary key and a foreign key. 12. a table is classified as a parent table and you want to drop and recreate it. how would you do this without affecting the children tables? A:disable the foreign key constraint to the parent, drop the table, recreate the table, enable the foreign key constraint. 13. explain the difference between archivelog mode and noarchivelog mode and the benefits and disadvantages to each. A:archivelog mode is a mode that you can put the database in for creating a backup of all transactions that have occurred in the database so that you can recover to any ball in time. noarchivelog mode is basically the absence of archivelog mode and has the disadvantage of not being able to recover to any ball in time. noarchivelog mode does have the advantage of not having to write transactions to an archive log and thus increases the performance of the database slightly. 14. what command would you use to create a backup control file? A:alter database backup control file to trace. 15. give the stages of instance startup to a usable state where normal users may access it. A:startup nomount - instance startup startup mount - the database is mounted startup open - the database is opened 16. what column differentiates the v$ views to the gv$ views and how? A:the inst_id column which indicates the instance in a rac environment the information came from. 17. how would you go about generating an explain plan?

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A:create a plan table with utlxplan.sql. use the explain plan set statement_id = 'tst1' into plan_table for a sql statement look at the explain plan with utlxplp.sql or utlxpls.sql 18. how would you go about increasing the buffer cache hit ratio? A:use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. if a change was necessary then i would use the alter system set db_cache_size command. 19. explain an ora-01555 A:you get this error when you get a snapshot too old within rollback. it can usually be solved by increasing the undo retention or increasing the size of rollbacks. You should also look at the logic involved in the application getting the error message. 20. explain the difference between $oracle_home and $oracle_base. A:oracle_base is the root directory for oracle. oracle_home located beneath oracle_base is where the oracle products reside. 12.INDEXES-ORACLE All About Indexes in Oracle What is an Index? A:An index is used to increase read access performance. A book, having an index, allows rapid access to a particular subject area within that book. Indexing a database table provides rapid location of specific rows within that table, where indexes are used to optimize the speed of access to rows. When indexes are not used or are not matched by SQL statements submitted to that database then a full table scan is executed. A full table scan will read all the data in a table to find a specific row or set of rows, this is extremely inefficient when there are many rows in the table. *It is often more efficient to full table scan small tables. The optimizer will often assess full table scan on small tables as being more efficient than reading both index and data space, particularly where a range scan rather than an exact match would be used against the index. An index of columns on a table contains a one-to-one ratio of rows between index and indexed table, excluding binary key groupings, a

more on this later. An index is effectively a separate table to that of the data table. Tables and indexes are often referred to as data and index spaces. An index contains the indexed columns plus a ROWID value for each of those column combination rows. When an index is searched through the indexed columns rather than all the data in the row of a table is scanned. The index space ROWID is then used to access the table row directly in the data space. An index row is generally much smaller than a table row, thus more index rows are stored in the same physical space, a block. As a result less of the database is accessed when using indexes as opposed to tables to search for data. This is the reason why indexes enhance performance. The Basic "How to" of Indexing A:There are a number of important factors with respect to efficient and effective creation and use of indexing.  The number of indexes per table.  The number of table columns to be indexed.  What datatypes are sensible for use in columns to be indexed?  Types of indexes from numerous forms of indexes available.  How does SQL behave with indexes?  What should be indexed?  What should not be indexed? Number of Indexes per Table Whenever a table is inserted into, updated or deleted from, all indexes plus the table must be updated. Thus if one places ten indexes onto a single table then every change to that table requires an effective change to a single table and ten indexes. The result is that performance will be substantially degraded since one insert requires eleven inserts to insert the new row into both data and index spaces. Be frugal with indexing and be conscious of the potential ill as well as the good effects produced by indexing. The general rule is that the more dynamic a table is the fewer indexes it should have.

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A dynamic table is a table changes constantly, such as a transactions table. Catalog tables on the other hand store information such as customer details; customers change a lot less often than invoices. Customer details are thus static in nature and over-indexing may be advantageous to performance. Number of Columns to Index Composite indexes are indexes made up of multiple columns. Minimize on the number of columns in a composite key. Create indexes with single columns. Composite indexes are often a requirement of traditional relational database table structures. With the advent of object-oriented application programming languages such as Java, sequence identifiers tend to be used to identify every row in every table uniquely. The result is single column indexes for every table. The only exceptions are generally manytomany join resolution entities. It may sometimes be better to exclude some of the lower-level or less relevant columns from the index since at that level there may not be much data, if there are not many rows to index it can be more efficient to read a group of rows from the data space. For instance, a composite index comprised of five columns could be reduced to the first three columns based on a limited number of rows traversed as a result of ignoring the last two columns. Look at your data carefully when constructing indexes. The more columns you add to a composite index the slower the search will be since there is a more complex requirement for that search and the indexes get physically larger. The benefit of indexes is that an index occupies less physical space than the data. If the index gets so large that it is as large as the data then it will become less efficient to read both the index and data spaces rather than just the data space. Most database experts recommend a maximum of three columns for composite keys. Datatypes of Index Columns Integers make the most efficient indexes. Try to always create indexes on columns with fixed length values. Avoid using VARCHAR2 and any object data types. Use integers if possible or fixed length, short strings. Also try to avaoid indexing on dates and floatingpoint values. If using dates be sure to use the internal representation or just the date, not the date and the time. Use integer generating sequences wherever possible to create consistently sequential values. a

Types of Indexes There are different types of indexes available in different databases. These different indexes are applicable under specific circumstances, generally for specific search patterns, for instance exact matches or range matches. The simplest form of indexing is no index at all, a heap structure. A heap structure is effectively a collection of data units, rows, which is completely unordered. The most commonly used indexed structure is a B tree (Binary Tree). A B tree index is best used for exact matches and range searches. Other methods of indexing exist. 1. Hashing algorithms produce a pre-calculated best guess on general row location and are best used for exact matches. 2. ISAM or Indexed Sequential Access Method indexes are not used in Oracle. 3. Bitmaps contain maps of zero's and 1's and can be highly efficient access methods for read-only data. 4. There are other types of indexing which involve clustering of data with indexes. In general every index type other than a B tree involves overflow. When an index is required to overflow it means that the index itself cannot be changed when rows are added, changed or removed. The result is inefficiency because a search to find overflowing data involves a search through originally indexed rows plus overflowing rows. Overflow index space is normally not ordered. A B tree index can be altered by changes to data. The only exception to a B tree index coping with data changes in Oracle is deletion of rows. When rows are deleted from a table, physical space previously used by the index for the deleted row is never reclaimed unless the index is rebuilt. Rebuilding of B tree indexes is far less common than that for other types of indexes since non-B tree indexes simply overflow when row changes are applied to them. Oracle uses has the following types of indexing available.  B tree index. A B tree is a binary tree. General all-round index and common in OLTP systems. An Oracle B tree index has three layers, the first two are branch node layers and the third, the lowest, contains leaf nodes. The branch nodes contain pointers to the lower level branch or leaf node. Leaf nodes contain index column values plus a ROWID pointer to the table row. The branch and leaf nodes are optimally arranged in the tree such that each branch will contain an equal number of branch or leaf nodes.

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 Bitmap index. Bitmap containing binary representations for each row. A zero implies that a row does not have a specified value and a 1 denotes that row having that value. Bitmaps are very susceptible to overflow in OLTP systems and should only be used for read-only data such as in Data Warehouses.  Function-Based index. Contains the result of an expression precalculated on each row in a table.  Index Organized Tables. Clusters index and data spaces together physically for a single table and orders the merged physical space in the order of the index, usually the primary key. An index organized table is a table as well as an index, the two are merged.  Clusters. Partial merge of index and data spaces, ordered by an index, not necessarily the primary key. A cluster is similar to an index organized table except that it can be built on a join (more than a single table). Clusters can be ordered using binary tree structures or hashing algorithms. A cluster could also be viewed as a table as well as an index since clustering partially merges index and data spaces.  Bitmap Join index. Creates a single bitmap for one table in a join.  Domain index. Specific to certain application types using contextual or spatial data, amongst others. Indexing Attributes Various types of indexes can have specific attributes or behaviors applied to them. These behaviors are listed below, some are Oracle specific and some are not.  Ascending or Descending. Indexes can be order in either way.  Uniqueness. Indexes can be unique or non-unique. Primary keys must be unique since a primary key uniquely identifies a row in a table referentially. Other columns such as names sometimes have unique constraints or indexes, or both, added to them.  Composites. A composite index is an index made up of more than one column in a table.  Compression. Applies to Btree indexes where duplicated prefix values are removed. Compression speeds up data retrieval but can slow down table changes.

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 Reverse keys. Bytes for all columns in the index are reversed, retaining the order of the columns. Reverse keys can help performance in clustered server environments (Oracle8i Parallel Server / RAC Oracle9i) by ensuring that changes to similar key values will be better physically spread. Reverse key indexing can apply to rows inserted into OLTP tables using sequence integer generators, where each number is very close to the previous number. When searching for and updating rows with sequence identifiers, where rows are searched for  Null values. Null values are generally not included in indexes.  Sorting (NOSORT). This option is Oracle specific and does not sort an index. This assumes that data space is physically ordered in the desired manner. What SQL does with Indexes A:In general a SQL statement will attempt to match the structure of itself to an index, the where clause ordering will attempt to match available indexes and use them if possible. If no index is matched then a full table scan will be executed. A table scan is extremely inefficient for anything but the smallest of tables. Obviously if a table is read sequentially, in physical order then an index is not required. A table does not always need an index. What to Index A:Use indexes where frequent queries are performed with where and order by clause matching the ordering of columns in those indexes. Use indexing generally on larger tables or multi-table, complex joins. Indexes are best created in the situations listed below.  Columns used in joins.  Columns used in where clauses.  Columns used in order by clauses.  In most relational databases the order-by clause is generally executed on the subset retrieved by the where clause, not the entire data space. This is not always unfortunately the case for Oracle. Traditionally the order-by clause should never include the columns contained in the where cause. The only case where the order-by clause will include columns contained in the where clause is the case of the where clause not matching any index in the database or a requirement a

for the order by clause to override the sort order of the where, typically in highly complex, multi-table joins.  The group-by clause can be enhanced by indexing when the range of values being grouped is small in relation to the number of rows in the table selected. What not to Index A:Indexes will degrade performance of inserts, updates and deletes, sometimes substantially.  Tables with a small number of rows.  Static tables.  Columns with a wide range of values.  Tables changed frequently and with a low amount of data retrieval.  Columns not used in data access query select statements. Tuning Oracle SQL Code and Using Indexes What is SQL Tuning? A:Tune SQL based on the nature of your application, OLTP or read-only Data Warehouse. OLTP applications have high volumes of concurrent transactions and are better served with exact match SQL where many transactions compete for small amounts of data. Read-only Data Warehouses require rapid access to large amounts of information at once and thus many records are accessed at once, either by many or a small number of sessions. The EXPLAIN PLAN command can be used to compare different versions of SQL statements, and tune your application SQL code as required. When tuning OLTP applications utilize sharing of SQL code in PL/SQL procedures and do not use triggers unless absolutely necessary. Triggers can cause problems such as self-mutating transactions where a table can expect a lock on a row already locked by the same transaction. This is because triggers do not allow transaction a

termination commands such as COMMIT and ROLLBACK. In short, do not use triggers unless absolutely necessary. The best approach to tuning of SQL statements is to seek out those statements consuming the greatest amount of resources (CPU, memory and I/O). The more often a SQL statement is executed the more finely it should be tuned. Additionally SQL statements executed many times more often than other SQL statements can cause issues withlocking. SQL code using bind variables will execute much faster than those not. Constant re-parsing of similar SQL code can over stress CPU time resources. Tuning is not necessarily a never-ending process but can be iterative. It is always best to take small steps and then assess improvements. Small changes are always more manageable and more easier to implement. Use the Oracle performance views plus tools such as TKPROF, tracing, Oracle Enterprise Manager, Spotlight, automated scripts and other tuning tools or packages which aid in monitoring and Oracle performance tuning. Detection of bottlenecks and SQL statements causing problem is as important asresolving those issues. In general tuning falls into three categories as listed below, in order of importance and performance impact. 1. Data model tuning. 2. SQL statement tuning. 3. Physical hardware and Oracle database configuration. Physical hardware and Oracle database configuration installation will, other than bottleneck resolution, generally only affect performance by between 10% and 20%. Most performance issues occur from poorly developed SQL code, with little attention to SQL tuning during development, probably causing around 80% of general system performance problems. Poor data model design can cause even more serious performance problems than SQL code but it is rare because data models are usually built more carefully than SQL code. It is a common problem that SQL code tuning is often left to DBA personnel. DBA people are often trained as Unix Administrators, SQL tuning is conceptually a programming skill; programming skills of Unix Administrators are generally very lowlevel,if present at all, and very different in skills requirements to that of SQL code. How to Tune SQL a

Indexing A:When building and restructuring of indexing never be afraid of removing unused indexes.The DBA should always be aware of where indexes are used and how. Oracle9i can automatically monitor index usage using the ALTER INDEX index name [NO]MONITORING USAGE; command with subsequent selection of the USED column from the V$OBJECT_USAGE column. Taking an already constructed application makes alterations of any kind much more complex. Pay most attention to indexes most often utilized. Some small static tables may not require indexes at all. Small static lookup type tables can be cached but will probably be force table-scanned by the optimizer anyway; table-scans may be adversely affected by the addition of unused superfluous indexes. Sometimes table-scans are faster than anything else. Consider the use of clustering, hashing, bitmaps and even index organized tables, only in Data Warehouses. Many installations use bitmaps in OLTP databases, this often a big mistake! If you have bitmap indexes in your OLTP database and are having performance problems, get rid of them! Oracle recommends the profligate use of function-based indexes, assuming of course there will not be too many of them. Do not allow too many programmers to create their indexes, especially not functionbased indexes, because you could end-up with thousands of indexes. Application developers tend to be unaware of what other developers are doing and create indexes specific to a particular requirement where indexes may be used in only one place. Some DBA control and approval process must be maintained on the creation of new indexes. Remember, every table change requires a simultaneous update to all indexes created based on that table. SQL Statement Reorganisation SQL statement reorganization encompasses factors as listed below, amongst others.  WHERE clause filtering and joining orders matching indexes.  Use of hints is not necessarily a good idea. The optimizer is probably smarter than you are.  Simplistic SQL statements and minimizing on table numbers in joins.

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 Use bind variables to minimize on re-parsing. Buying lots of expensive RAM and sizing your shared pool and database buffer cache to very large values may make performance worse. Firstly, buffer cache reads are not as fast as you might think. Secondly, a large SQL parsing shared pool, when not using bind variables in SQL code, will simply fill up and take longer for every subsequent SQL statement to search.  Oracle9i has adopted various SQL ANSI standards. The ANSI join syntax standard could cause SQL code performance problems. The most effective tuning approach to tuning Oracle SQL code is to remove rows from joins using where clause filtering prior to joining multiple tables, obviously the larger tables, requiring the fewest rows should be filtered first. ANSI join syntax applies joins prior to where clause filtering; this could cause major performance problems. Nested subquery SQL statements can be effective under certain circumstances. However, nesting of SQL statements increases the level of coding complexity and if sometimes looping cursors can be utilized in PL/SQL procedures, assuming the required SQL is not completely adhoc. Avoid ad-hoc SQL if possible. Any functionality, not necessarily business logic, is always better provided at the application level. Business logic, in the form of referential integrity, is usually best catered for in Oracle using primary and foreign key constraints and explicitly created indexes. Nested subquery SQL statements can become over complicated and impossible for even the most brilliant coder to tune to peak efficiency. The reason for this complexity could lie in an over-Normalized underlying data model. In general use of subqueries is a very effective approach to SQL code performance tuning. However, the need to utilize intensive, multi-layered subquery SQL code is often a symptom of a poor data model due to requirements for highly complex SQL statement joins. Some Oracle Tricks Use [NOT] EXISTS Instead of [NOT] IN In the example below the second SQL statement utilizes an index in the subquery because of the use of EXISTS in the second query as opposed to IN. IN will build a set first and EXISTS will not. IN will not utilize indexes whereas EXISTS will. SELECT course_code, name FROM student

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WHERE course_code NOT IN (SELECT course_code FROM maths_dept); SELECT course_code, name FROM student WHERE NOT EXISTS (SELECT course_code FROM maths_dept WHERE maths_dept.course_code = student.course_code); In the example below the nesting of the two queries could be reversed depending on which table has more rows. Also if the index is not used or not available, reversal of the subquery is required if tableB has significantly more rows than tableA. DELETE FROM tableA WHERE NOT EXISTS (SELECT columnB FROM tableB WHERE tableB.columnB = tableA.columnA); Use of value lists with the IN clause could indicate a missing entity. Also that missing entity is probably static in nature and can potentially be cached, although caching causing increased data buffer size requirements is not necessarily a sensible solution. SELECT country FROM countries WHERE continent IN ('africa','europe','north america'); Equijoins and Column Value Transformations AND and = predicates are the most efficient. Avoid transforming of column values in any form, anywhere in a SQL statement, for instance as shown below. SELECT * FROM WHERE TO_NUMBER(BOX_NUMBER) = 94066; And the example below is really bad! Typically indexes should not be placed on descriptive fields such as names. A function-based index would be perfect in this case but would probably be unnecessary if the data model and the data values were better organized. SELECT * FROM table1, table2

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WHERE UPPER(SUBSTR(table1.name,1,1)) = UPPER(SUBSTR(table2.name,1,1)); Transforming literal values is not such a problem but application of a function to a column within a table in a where clause will use a tablescan regardless of the presence of indexes. When using a functionbased index the index is the result of the function which the optimizer will recognize and utilize for subsequent SQL statements. Datatypes Try not to use mixed datatypes by setting columns to appropriate types in the first place. If mixing of datatypes is essential do not assume implicit type conversion because it will not always work, and implicit type conversions can cause indexes not to be used. Function-based indexes can be used to get around type conversion problems but this is not the most appropriate use of function-based indexes. If types must be mixed, try to place type conversion onto explicit values and not columns. For instance, as shownbelow. WHERE zip = TO_NUMBER('94066') as opposed to WHERE TO_CHAR(zip) = '94066' The DECODE Function The DECODE function will ignore indexes completely. DECODE is very useful in certain circumstances where nested looping cursors can become extremely complex. DECODE is intended for specific requirements and is not intended to be used prolifically, especially not with respect to type conversions. Most SQL statements containing DECODE function usage can be altered to use explicit literal selection criteria perhaps using separate SELECT statements combined with UNION clauses. Also Oracle9i contains a CASE statement which is much more versatile than DECODE and may be much more efficient. Join Orders Always use indexes where possible, this applies to all tables accessed in a join, both those in the driving and nested subqueries. Use indexes between parent and child nested subqueries in order to utilize indexes across a join. A common error is that of accessing a single row from the driving table using an index and then to access all rows from a nested subquery table where an index can be used in the nested subquery table based on the rowretrieved by the driving table.

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Put where clause filtering before joins, especially for large tables where only a few rows are required. Try to use indexes fetching the minimum number of rows. The order in which tables are accessed in a query is very important. Generally a SQL statement is parsed from toptobottom and from left-to-right. The further into the join or SQL statement, then the fewer rows should be accessed. Even consider constructing a SQL statement based on the largest table being the driving table even if that largest table is not the logical driver of the SQL statement. When a join is executed, each join will overlay the result of the previous part of the join, effectively each section (based on each table) is executed sequentially. In the example below table1 has the most rows and table3 has the fewest rows. SELECT * FROM table1, table2, table3 WHERE table1.index = table2.index AND table1.index = table3.index; Hints Use them? Perhaps. When circumstances force their use. Generally the optimizer will succeed where you will not. Hints allow, amongst many other things, forcing of index usage rather than full table-scans. The optimizer will generally find full scans faster with small tables and index usage faster with large tables. Therefore if row numbers and the ratios of row between tables are known then using hints will probably make performance worse. One specific situation where hints could help are generic applications where rows in specific tables can change drastically depending on the installation. However, once again the optimizer may still be more capable than any programmer or DBA. Use INSERT, UPDATE and DELETE ... RETURNING When values are produced and contained in insert or update statements, such as new sequence numbers or expression results, and those values are required in the same transaction by following SQL statements, the values can be returned into variables and used later without recalculation of expression being required. This tactic would be used in PL/SQL and anonymous procedures. Examples are shown below. INSERT INTO table1 VALUES (test_id.nextval, 'Jim Smith', '100.12', 5*10) RETURNING col1, col4 * 2 INTO val1, val2; UPDATE table1 SET name = 'Joe Soap' a

WHERE col1 = :val1 AND col2 = :val2; DELETE FROM table1 RETURN value INTO :array; Triggers Simplification or complete disabling and removal of triggers is advisable. Triggers are very slow and can cause many problems, both performance related and can even cause serious locking and database errors. Triggers were originally intended for messaging and are not intended for use as rules triggered as a result of a particular event. Other databases have full-fledged rule systems aiding in the construction of Rule-Based Expert systems. Oracle triggers are more like database events than event triggered rules causing other potentially recursive events to occur. Never use triggers to validate referential integrity. Try not to use triggers at all. If you do use triggers and have performance problems, their removal and recoding into stored procedures, constraints or application code could solve a lot of your problems. Data Model Restructuring Data restructuring involves partitioning, normalization and even denormalization. Oracle recommends avoiding the use of primary and foreign keys for validation of referential integrity, and suggests validating referential integrity in application code. Applicationcode is more prone to error since it changes much faster. Avoiding constraintbased referential is not necessarily the best solution. Referential integrity can be centrally controlled and altered in a single place in the database. Placing referential integrity in application code is less efficient due to increased network traffic and requires more code to be maintained in potentially many applications. All foreign keys must have indexes explicitly created and these indexes will often be used in general application SQL calls, other than just for validation of referential integrity. Oracle does not create internal indexes when creating foreign reference keys. In fact Oracle recommends that indexes should be explicitly created for all primary, foreign and unique hey constraints. Foreign keys not indexed using the CREATE INDEX statement can cause table locks on the table containing the foreign key. It is highly likely that foreign keys will often be used by the optimizer in SQL statement filtering where clauses, if the data model and the application are consistent with each other structurally, which should be the case.

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Views Do not use views as a basis for SQL statements taking a portion of the rows defined by that view. Views were originally intended for security and access privileges. No matter what where clause is applied to a view the entire view will always be executed first. On the same basis also avoid things such as SELECT * FROM …, GROUP BY clauses and aggregations such as DISTINCT. DISTINCT will always select all rows first. Do not create new entities using joined views, it is better to create those intersection view joins as entities themselves; this applies particularly in the case of many-to-many relationships. Also Data Warehouses can benefit from materialized views which are views actually containing data, refreshed by the operator at a chosen juncture. Maintenance of Current Statistics and Cost Based Optimization Maintain current statistics as often as possible, this can be automated. Cost-based optimization, using statistics, is much more efficient than rule-based optimization. Regeneration and Coalescing of Indexes Indexes subjected to constant DML update activity can become skewed and thus become less efficient over a period of time. Use of Oracle Btree indexes implies that when a value is searched for within the tree, a series of comparisons are made in order to depth-firsttraverse down through the tree until the appropriate value is found. Oracle Btree indexes are usually only three levels, requiring three hits on the index to find a resulting ROWID pointer to a table row. Index searches, even into very large tables, especially unique index hits, not index range scans, can be incredibly fast.In some circumstances constant updating of a binary tree can cause the tree to becomemore heavily loaded in some parts or skewed. Thus some parts of the tree require more intensive searching which can be largely fruitless. Indexes should sometimes be rebuilt where the binary tree is regenerated from scratch, this can be done online in Oracle9i, as shown below. ALTER INDEX index name REBUILD ONLINE; Coalescing of indexes is a more physical form of maintenance where physical space chunks which are fragmented. Index fragmentation is usually a result of massivedeletions from table, at once or over time. Oracle Btree indexes do not reclaim physical space as a result of row deletions. This can cause serious performance problems as a result of a

fruitless searches and very large index files where much of the index space is irrelevant. The command shown below will do a lot less than rebuilding but it can help. If PCTINCREASE is not set to zero for the index then extents could vary in size greatly and not be reusable. In that the only option is rebuilding. ALTER INDEX index name REBUILD COALESCE NOLOGGING; 17.DBA BASIC QUESTIONS 1)What are the prerequisites for connecting to a database > 1) oracle net services should be available in both server and client. 2) listner should up and running. In case of remote connection.. [oracle listiner starts up a dedicated server process and passes the server protocal adress to client using that address the clients connect to the server. Once the connection is established the listiner connection is terminated] *********************************************************************** [AND] 1) check wether database server is installed on server or not. 2) client software should be installed on client machine. 3) check database and client are running on the same network or not. (with the help of ping 4) ensure thar oracle listiner is up and running 5) connect to server using server protocal address 2) Create a User "TESTAPPS" identified by "TESTAPPS" > create user identified by ; 3) Connect to DB using TESTAPPS from DB Node and MT Node > first give grant options to the user.... grant connect,resource to ; 4)How do you identify remote connections on a DB Server > ps -ef|grep -i local [where local=no it is a remote connection... in the os level] 5)How do you identify local connections on a DB Server > ps -ef|grep -i local [where local=yes it is a local connection... in the os level] 6)Can you connect remotely as a user on DB Server. If so, how? > /@ [with the help of connecting string] 7)Do you need to acess to DB Server to connect to a system schema? > NO , just knowing the username&password u connect from the a

client... 8)What is the difference between "SYS" & "SYSTEM" Schema > SYS is a super most user... SYS has additional roles sysdba, sysoper SYS can do only startup, shudown options > SYSTEM schema has owns certain additional data dictonary tables.. SYSTEM donot use startup and shutdown options.... 9)What are the roles/priviliges for a "SYS" Schema >***ROLES***[select granted_role from dba_role_privs where grantee='SYS'] IMP_FULL_DATABASE, DELETE_CATALOG_ROLE, RECOVERY_CATALOG_OWNER, DBA, EXP_FULL_DATABASE, HS_ADMIN_ROLE, AQ_ADMINISTRATOR_ROLE, OEM_MONITOR, RESOURCE, EXECUTE_CATALOG_ROLE, LOGSTDBY_ADMINISTRATOR, AQ_USER_ROLE, SCHEDULER_ADMIN, CONNECT, SELECT_CATALOG_ROLE, GATHER_SYSTEM_STATISTICS, OEM_ADVISOR ***PRIVILAGES****[select privileges from dba_sys_privs where grantee='SYS';] CREATE ANY RULE CREATE ANY EVALUATION CONTEXT MANAGE ANY QUEUE EXECUTE ANY PROCEDURE ALTER ANY RULE CREATE RULE SET EXECUTE ANY EVALUATION CONTEXT INSERT ANY TABLE SELECT ANY TABLE LOCK ANY TABLE UPDATE ANY TABLE DROP ANY RULE SET ENQUEUE ANY QUEUE EXECUTE ANY TYPE CREATE RULE ALTER ANY EVALUATION CONTEXT CREATE EVALUATION CONTEXT ANALYZE ANY EXECUTE ANY RULE DROP ANY EVALUATION CONTEXT EXECUTE ANY RULE SET a

ALTER ANY RULE SET DEQUEUE ANY QUEUE DELETE ANY TABLE DROP ANY RULE CREATE ANY RULE SET SELECT ANY SEQUENCE 10)What are the role/privileges for a SYSTEM Schema > **ROLES** [select granted_role from dba_role_privs where grantee='SYSTEM';] AQ_ADMINISTRATOR_ROLE DBA >**PRIVILEGES*** [select privilege from dba_sys_privs where grantee='SYSTEM';] GLOBAL QUERY REWRITE CREATE MATERIALIZED VIEW CREATE TABLE UNLIMITED TABLESPACE SELECT ANY TABLE 11)What is the difference between SYSDBA & DBA > SYSDBA has startup and shutdown options > DBA has no startup and shutdown options 12)What is the difference between X$ , V$ ,V_$,GV$ > X$ is permenent views > GV$ are used in RAC environment.... > v$, V_$ are the temporary views which exist during the run time.... 13)How do you verify whether your DB is a single node or Multinode > sho parameter cluster; its show false ..means single node. 14)From MT connect to db using "connect / as sysdba" > /as sysdba cannot connect to database from MT ... or u can connect to DB from MT by creating password file 15)Is a Listener required to be up and running for a Local Connection > NO 16)Is a Listener required to be up and running for a remote Connection > YES a

17)How do you verify the Background processes running from the Database > desc v$bgprocess select * from v$bgprocess; 18)How do you verify whether a init.ora parameter is modifiable or not. > desc v$parameter select name,value,ISSES_MODIFIABLE,ISSYS_MODIFIABLE,ISINSTANCE_MODIFI ABLE from v$parameter; 19)What are the various ways to modify an init.ora parameter > Two ways ... static & dynamic static... editing text in the init.ora file...... dynamic... alter system set = scope=both (or) scope=spfile (or) scope=memory; 20)Why is init.ora file required > For starting instance... defining the parameters values...[memory structures] defining the control files location.... 21)Why is a DB required to be in archive Log > To recover the database 22)List the total No. of objects available in an apps database with respect to a owner,object type,status 23)When an DB is being started where is the information being recorded > Alert logfile..... 24)What is the information that is being recorded at the time of db Start .... 25)What is the difference between instance and Database > INSTANCE is group of memory structures and background processes, its a volatile memory. > DATABASE is physical memory.. collection of C, R, D files..... 26)How is an instance created > when ever issue the startup command.... a

server process reads the inti.ora file... and its internally reads the SGA allocated size, memory structures values, parametrs values... using these parameters instance is created 27)What are the files essential to start an instance > init.ora file.... and internally its parameters... > if its remote connection needs init.orafile & password file.. 28)While the instance is being created can users connect to Database. > NO, normal users cannot connect the database.. only sys user can connect > normal users has no privileges... to connect the database in nomunt & mount stages.... 29)Startup an instance. Connect as user Testapps. Verify the data dictionary table dba_data_files. What are the data dictionary objects that can be viewed > normal users cannot connect to database... 30)After completing step 31, exit out of sql session, connect as "apps" 31)When the instance is created how many Unix processes are created and how do you view them > startup nomount ps -ux.... or alert log file... PMON started with pid=2, OS id=4482 PSP0 started with pid=3, OS id=4484 MMAN started with pid=4, OS id=4486 DBW0 started with pid=5, OS id=4488 LGWR started with pid=6, OS id=4490 CKPT started with pid=7, OS id=4492 SMON started with pid=8, OS id=4494 RECO started with pid=9, OS id=4496 MMON started with pid=10, OS id=4498 MMNL started with pid=11, OS id=4500 ten background process and one server and cleint process...total 12.. 32)When the database is mounted how many Unix processes are created and how do you view them > ps -ux or alert logfile... > In mount stage (alter database mount) no extra process are not created.... 33)How do you mount a database after an instance is created. What a

are the messages recorded while changing to mount stage > alter database mount > Setting recovery target incarnation to 1 > Successful mount of redo thread 1, with mount id 4186671158 > Database mounted in Exclusive Mode > Completed: alter database mount 34) What are the data dictionary objects that can be viewed in a mount stage. 35)How do you open a database after an instance is mounted. What are the messages recorded while changing to open stage > alter database open > opening redolog files. > Successful open of redo files. > MTTR advisory is disabled because FAST_START_MTTR_TARGET is not set > SMON: enabling cache recovery > Successfully onlined Undo Tablespace 1 > SMON: enabling tx recovery > Database Characterset is US7ASCII > Completed: alter database open ************************************************************************ **** No answer qus:13, 22, 29, 30, 34 18.Tablespaces-Oracle DBA Tablespaces, Datafiles, and Control Files This chapter describes tablespaces, the primary logical database structures of any Oracle database, and the physical datafiles that correspond to each tablespace. This chapter contains the following topics: * * * *

Introduction to Tablespaces, Datafiles, and Control Files Overview of Tablespaces Overview of Datafiles Overview of Control Files

Introduction to Tablespaces, Datafiles, and Control Files Oracle stores data logically in tablespaces and physically in datafiles associated with the corresponding tablespace. Figure 3-1 illustrates a

this relationship.

Figure 3-1 Datafiles and Tablespaces Your browser may not support display of this image. Description of "Figure 3-1 Datafiles and Tablespaces" Databases, tablespaces, and datafiles are closely related, but they have important differences: * An Oracle database consists of one or more logical storage units called tablespaces, which collectively store all of the database's data. * Each tablespace in an Oracle database consists of one or more files called datafiles, which are physical structures that conform to the operating system in which Oracle is running. * A database's data is collectively stored in the datafiles that constitute each tablespace of the database. For example, the simplest Oracle database would have one tablespace and one datafile. Another database can have three tablespaces, each consisting of two datafiles (for a total of six datafiles). Oracle-Managed Files Oracle-managed files eliminate the need for you, the DBA, to directly manage the operating system files comprising an Oracle database. You specify operations in terms of database objects rather than filenames. Oracle internally uses standard file system interfaces to create and delete files as needed for the following database structures: * Tablespaces * Redo log files * Control files a

Through initialization parameters, you specify the file system directory to be used for a particular type of file. Oracle then ensures that a unique file, an Oracle-managed file, is created and deleted when no longer needed. Allocate More Space for a Database The size of a tablespace is the size of the datafiles that constitute the tablespace. The size of a database is the collective size of the tablespaces that constitute the database. You can enlarge a database in three ways: * Add a datafile to a tablespace * Add a new tablespace * Increase the size of a datafile When you add another datafile to an existing tablespace, you increase the amount of disk space allocated for the corresponding tablespace. Figure 3-2 illustrates this kind of space increase.

Figure 3-2 Enlarging a Database by Adding a Datafile to a Tablespace

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Description of "Figure 3-2 Enlarging a Database by Adding a Datafile to a Tablespace" Alternatively, you can create a new tablespace (which contains at least one additional datafile) to increase the size of a database. Figure 3-3 illustrates this.

Figure 3-3 Enlarging a Database by Adding a New Tablespace Your browser may not support display of this image. Description of "Figure 3-3 Enlarging a Database by Adding a New Tablespace" The third option for enlarging a database is to change a datafile's size or let datafiles in existing tablespaces grow dynamically as more space is needed. You accomplish this by altering existing files or by adding files with dynamic extension properties. Figure 3-4 illustrates this.

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Figure 3-4 Enlarging a Database by Dynamically Sizing Datafiles Your browser may not support display of this image. Description of "Figure 3-4 Enlarging a Database by Dynamically Sizing Datafiles" Overview of Tablespaces A database is divided into one or more logical storage units called tablespaces. Tablespaces are divided into logical units of storage called segments, which are further divided into extents. Extents are a collection of contiguous blocks. This section includes the following topics about tablespaces: * * * * * * * * * * * *

Bigfile Tablespaces The SYSTEM Tablespace The SYSAUX Tablespace Undo Tablespaces Default Temporary Tablespace Using Multiple Tablespaces Managing Space in Tablespaces Multiple Block Sizes Online and Offline Tablespaces Read-Only Tablespaces Temporary Tablespaces for Sort Operations Transport of Tablespaces Between Databases

See Also: o Chapter 2, "Data Blocks, Extents, and Segments" for more information about segments and extents o Oracle Database Administrator's Guide for detailed information on creating and configuring tablespaces Bigfile Tablespaces Oracle lets you create bigfile tablespaces. This allows Oracle Database to contain tablespaces made up of single large files rather than numerous smaller ones. This lets Oracle Database utilize the ability of 64-bit systems to create and manage ultralarge files. The consequence of this is that Oracle Database can now scale up to 8 exabytes in size. With Oracle-managed files, bigfile tablespaces make datafiles completely transparent for users. In other words, you can perform a

operations on tablespaces, rather than the underlying datafile. Bigfile tablespaces make the tablespace the main unit of the disk space administration, backup and recovery, and so on. Bigfile tablespaces also simplify datafile management with Oracle-managed files and Automatic Storage Management by eliminating the need for adding new datafiles and dealing with multiple files. The system default is to create a smallfile tablespace, which is the traditional type of Oracle tablespace. The SYSTEM and SYSAUX tablespace types are always created using the system default type. Bigfile tablespaces are supported only for locally managed tablespaces with automatic segment-space management. There are two exceptions: locally managed undo and temporary tablespaces can be bigfile tablespaces, even though their segments are manually managed. An Oracle database can contain both bigfile and smallfile tablespaces. Tablespaces of different types are indistinguishable in terms of execution of SQL statements that do not explicitly refer to datafiles. You can create a group of temporary tablespaces that let a user consume temporary space from multiple tablespaces. A tablespace group can also be specified as the default temporary tablespace for the database. This is useful with bigfile tablespaces, where you could need a lot of temporary tablespace for sorts. Benefits of Bigfile Tablespaces * Bigfile tablespaces can significantly increase the storage capacity of an Oracle database. Smallfile tablespaces can contain up to 1024 files, but bigfile tablespaces contain only one file that can be 1024 times larger than a smallfile tablespace. The total tablespace capacity is the same for smallfile tablespaces and bigfile tablespaces. However, because there is limit of 64K datafiles for each database, a database can contain 1024 times more bigfile tablespaces than smallfile tablespaces, so bigfile tablespaces increase the total database capacity by 3 orders of magnitude. In other words, 8 exabytes is the maximum size of the Oracle database when bigfile tablespaces are used with the maximum block size (32 k). * Bigfile tablespaces simplify management of datafiles in ultra large databases by reducing the number of datafiles needed. You can also adjust parameters to reduce the SGA space required for datafile information and the size of the control file. * They simplify database management by providing datafile transparency. a

Considerations with Bigfile Tablespaces * Bigfile tablespaces are intended to be used with Automatic Storage Management or other logical volume managers that support dynamically extensible logical volumes and striping or RAID. * Avoid creating bigfile tablespaces on a system that does not support striping because of negative implications for parallel execution and RMAN backup parallelization. * Avoid using bigfile tablespaces if there could possibly be no free space available on a disk group, and the only way to extend a tablespace is to add a new datafile on a different disk group. * Using bigfile tablespaces on platforms that do not support large file sizes is not recommended and can limit tablespace capacity. Refer to your operating system specific documentation for information about maximum supported file sizes. * Performance of database opens, checkpoints, and DBWR processes should improve if data is stored in bigfile tablespaces instead of traditional tablespaces. However, increasing the datafile size might increase time to restore a corrupted file or create a new datafile. The SYSTEM Tablespace Every Oracle database contains a tablespace named SYSTEM, which Oracle creates automatically when the database is created. The SYSTEM tablespace is always online when the database is open. To take advantage of the benefits of locally managed tablespaces, you can create a locally managed SYSTEM tablespace, or you can migrate an existing dictionary managed SYSTEM tablespace to a locally managed format. In a database with a locally managed SYSTEM tablespace, dictionary managed tablespaces cannot be created. It is possible to plug in a dictionary managed tablespace using the transportable feature, but it cannot be made writable. Note: If a tablespace is locally managed, then it cannot be reverted back to being dictionary managed. The Data Dictionary The SYSTEM tablespace always contains the data dictionary tables for the entire database. The data dictionary tables are stored in datafile 1. a

PL/SQL Program Units Description All data stored on behalf of stored PL/SQL program units (that is, procedures, functions, packages, and triggers) resides in the SYSTEM tablespace. If the database contains many of these program units, then the database administrator must provide the space the units need in the SYSTEM tablespace. The SYSAUX Tablespace The SYSAUX tablespace is an auxiliary tablespace to the SYSTEM tablespace. Many database components use the SYSAUX tablespace as their default location to store data. Therefore, the SYSAUX tablespace is always created during database creation or database upgrade. The SYSAUX tablespace provides a centralized location for database metadata that does not reside in the SYSTEM tablespace. It reduces the number of tablespaces created by default, both in the seed database and in user-defined databases. During normal database operation, the Oracle database server does not allow the SYSAUX tablespace to be dropped or renamed. Transportable tablespaces for SYSAUX is not supported. Note: If the SYSAUX tablespace is unavailable, such as due to a media failure, then some database features might fail. Undo Tablespaces Undo tablespaces are special tablespaces used solely for storing undo information. You cannot create any other segment types (for example, tables or indexes) in undo tablespaces. Each database contains zero or more undo tablespaces. In automatic undo management mode, each Oracle instance is assigned one (and only one) undo tablespace. Undo data is managed within an undo tablespace using undo segments that are automatically created and maintained by Oracle. When the first DML operation is run within a transaction, the transaction is bound (assigned) to an undo segment (and therefore to a transaction table) in the current undo tablespace. In rare circumstances, if the instance does not have a designated undo tablespace, the transaction binds to the system undo segment.

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Caution: Do not run any user transactions before creating the first undo tablespace and taking it online. Each undo tablespace is composed of a set of undo files and is locally managed. Like other types of tablespaces, undo blocks are grouped in extents and the status of each extent is represented in the bitmap. At any point in time, an extent is either allocated to (and used by) a transaction table, or it is free. You can create a bigfile undo tablespace. Creation of Undo Tablespaces A database administrator creates undo tablespaces individually, using the CREATE UNDO TABLESPACE statement. It can also be created when the database is created, using the CREATE DATABASE statement. A set of files is assigned to each newly created undo tablespace. Like regular tablespaces, attributes of undo tablespaces can be modified with the ALTER TABLESPACE statement and dropped with the DROP TABLESPACE statement. Note: An undo tablespace cannot be dropped if it is being used by any instance or contains any undo information needed to recover transactions. Assignment of Undo Tablespaces You assign an undo tablespace to an instance in one of two ways: * At instance startup. You can specify the undo tablespace in the initialization file or let the system choose an available undo tablespace. * While the instance is running. Use ALTER SYSTEM SET UNDO_TABLESPACE to replace the active undo tablespace with another undo tablespace. This method is rarely used. You can add more space to an undo tablespace by adding more datafiles to the undo tablespace with the ALTER TABLESPACE statement. You can have more than one undo tablespace and switch between them. Use the Database Resource Manager to establish user quotas for undo tablespaces. You can specify the retention period for undo a

information See Also: Tablespace When the SYSTEM tablespace is locally managed, you must define at least one default temporary tablespace when creating a database. A locally managed SYSTEM tablespace cannot be used for default temporary storage. If SYSTEM is dictionary managed and if you do not define a default temporary tablespace when creating the database, then SYSTEM is still used for default temporary storage. However, you will receive a warning in ALERT.LOG saying that a default temporary tablespace is recommended and will be necessary in future releases. How to Specify a Default Temporary Tablespace Specify default temporary tablespaces when you create a database, using the DEFAULT TEMPORARY TABLESPACE extension to the CREATE DATABASE statement. If you drop all default temporary tablespaces, then the SYSTEM tablespace is used as the default temporary tablespace. You can create bigfile temporary tablespaces. A bigfile temporary tablespaces uses tempfiles instead of datafiles. Note: You cannot make a default temporary tablespace permanent or take it offline. Using Multiple Tablespaces A very small database may need only the SYSTEM tablespace; however, Oracle recommends that you create at least one additional tablespace to store user data separate from data dictionary information. This gives you more flexibility in various database administration operations and reduces contention among dictionary objects and schema objects for the same datafiles. You can use multiple tablespaces to perform the following tasks: * Control disk space allocation for database data * Assign specific space quotas for database users * Control availability of data by taking individual tablespaces online or a

offline * Perform partial database backup or recovery operations * Allocate data storage across devices to improve performance A database administrator can use tablespaces to do the following actions: * Create new tablespaces * Add datafiles to tablespaces * Set and alter default segment storage settings for segments created in a tablespace * Make a tablespace read only or read/write * Make a tablespace temporary or permanent * Rename tablespaces * Drop tablespaces Managing Space in Tablespaces Tablespaces allocate space in extents. Tablespaces can use two different methods to keep track of their free and used space: * Locally managed tablespaces: Extent management by the tablespace * Dictionary managed tablespaces: Extent management by the data dictionary When you create a tablespace, you choose one of these methods of space management. Later, you can change the management method with the DBMS_SPACE_ADMIN PL/SQL package. Note: If you do not specify extent management when you create a tablespace, then the default is locally managed. Locally Managed Tablespaces A tablespace that manages its own extents maintains a bitmap in each datafile to keep track of the free or used status of blocks in that datafile. Each bit in the bitmap corresponds to a block or a group of blocks. When an extent is allocated or freed for reuse, Oracle changes the bitmap values to show the new status of the blocks. These changes do not generate rollback information because they do not update tables in the data dictionary (except for special cases such as tablespace quota information). Locally managed tablespaces have the following advantages over a

dictionary managed tablespaces: * Local management of extents automatically tracks adjacent free space, eliminating the need to coalesce free extents. * Local management of extents avoids recursive space management operations. Such recursive operations can occur in dictionary managed tablespaces if consuming or releasing space in an extent results in another operation that consumes or releases space in a data dictionary table or rollback segment. The sizes of extents that are managed locally can be determined automatically by the system. Alternatively, all extents can have the same size in a locally managed tablespace and override object storage options. The LOCAL clause of the CREATE TABLESPACE or CREATE TEMPORARY TABLESPACE statement is specified to create locally managed permanent or temporary tablespaces, respectively. Segment Space Management in Locally Managed Tablespaces When you create a locally managed tablespace using the CREATE TABLESPACE statement, the SEGMENT SPACE MANAGEMENT clause lets you specify how free and used space within a segment is to be managed. Your choices are: * AUTO This keyword tells Oracle that you want to use bitmaps to manage the free space within segments. A bitmap, in this case, is a map that describes the status of each data block within a segment with respect to the amount of space in the block available for inserting rows. As more or less space becomes available in a data block, its new state is reflected in the bitmap. Bitmaps enable Oracle to manage free space more automatically; thus, this form of space management is called automatic segment-space management. Locally managed tablespaces using automatic segment-space management can be created as smallfile (traditional) or bigfile tablespaces. AUTO is the default. * MANUAL This keyword tells Oracle that you want to use free lists for managing free space within segments. Free lists are lists of data blocks that have space available for inserting rows. a

Dictionary Managed Tablespaces If you created your database with an earlier version of Oracle, then you could be using dictionary managed tablespaces. For a tablespace that uses the data dictionary to manage its extents, Oracle updates the appropriate tables in the data dictionary whenever an extent is allocated or freed for reuse. Oracle also stores rollback information about each update of the dictionary tables. Because dictionary tables and rollback segments are part of the database, the space that they occupy is subject to the same space management operations as all other data. Multiple Block Sizes Oracle supports multiple block sizes in a database. The standard block size is used for the SYSTEM tablespace. This is set when the database is created and can be any valid size. You specify the standard block size by setting the initialization parameter DB_BLOCK_SIZE. Legitimate values are from 2K to 32K. In the initialization parameter file or server parameter, you can configure subcaches within the buffer cache for each of these block sizes. Subcaches can also be configured while an instance is running. You can create tablespaces having any of these block sizes. The standard block size is used for the system tablespace and most other tablespaces. Note: All partitions of a partitioned object must reside in tablespaces of a single block size. Multiple block sizes are useful primarily when transporting a tablespace from an OLTP database to an enterprise data warehouse. This facilitates transport between databases of different block sizes. Online and Offline Tablespaces A database administrator can bring any tablespace other than the SYSTEM tablespace online (accessible) or offline (not accessible) whenever the database is open. The SYSTEM tablespace is always online when the database is open because the data dictionary must always be available to Oracle. A tablespace is usually online so that the data contained within it is a

available to database users. However, the database administrator can take a tablespace offline for maintenance or backup and recovery purposes. Bringing Tablespaces Offline When a tablespace goes offline, Oracle does not permit any subsequent SQL statements to reference objects contained in that tablespace. Active transactions with completed statements that refer to data in that tablespace are not affected at the transaction level. Oracle saves rollback data corresponding to those completed statements in a deferred rollback segment in the SYSTEM tablespace. When the tablespace is brought back online, Oracle applies the rollback data to the tablespace, if needed. When a tablespace goes offline or comes back online, this is recorded in the data dictionary in the SYSTEM tablespace. If a tablespace is offline when you shut down a database, the tablespace remains offline when the database is subsequently mounted and reopened. You can bring a tablespace online only in the database in which it was created because the necessary data dictionary information is maintained in the SYSTEM tablespace of that database. An offline tablespace cannot be read or edited by any utility other than Oracle. Thus, offline tablespaces cannot be transposed to other databases. Oracle automatically switches a tablespace from online to offline when certain errors are encountered. For example, Oracle switches a tablespace from online to offline when the database writer process, DBWn, fails in several attempts to write to a datafile of the tablespace. Users trying to access tables in the offline tablespace receive an error. If the problem that causes this disk I/O to fail is media failure, you must recover the tablespace after you correct the problem. Use of Tablespaces for Special Procedures If you create multiple tablespaces to separate different types of data, you take specific tablespaces offline for various procedures. Other tablespaces remain online, and the information in them is still available for use. However, special circumstances can occur when tablespaces are taken offline. For example, if two tablespaces are used to separate table data from index data, the following is true: * If the tablespace containing the indexes is offline, then queries can still access table data because queries do not require an index to access the table data. a

* If the tablespace containing the tables is offline, then the table data in the database is not accessible because the tables are required to access the data. If Oracle has enough information in the online tablespaces to run a statement, it does so. If it needs data in an offline tablespace, then it causes the statement to fail. Read-Only Tablespaces The primary purpose of read-only tablespaces is to eliminate the need to perform backup and recovery of large, static portions of a database. Oracle never updates the files of a read-only tablespace, and therefore the files can reside on read-only media such as CD-ROMs or WORM drives. Note: Because you can only bring a tablespace online in the database in which it was created, read-only tablespaces are not meant to satisfy archiving requirements. Read-only tablespaces cannot be modified. To update a read-only tablespace, first make the tablespace read/write. After updating the tablespace, you can then reset it to be read only. Because read-only tablespaces cannot be modified, and as long as they have not been made read/write at any point, they do not need repeated backup. Also, if you need to recover your database, you do not need to recover any read-only tablespaces, because they could not have been modified. Temporary Tablespaces for Sort Operations You can manage space for sort operations more efficiently by designating one or more temporary tablespaces exclusively for sorts. Doing so effectively eliminates serialization of space management operations involved in the allocation and deallocation of sort space. A single SQL operation can use more than one temporary tablespace for sorting. For example, you can create indexes on very large tables, and the sort operation during index creation can be distributed across multiple tablespaces. All operations that use sorts, including joins, index builds, ordering, computing aggregates (GROUP BY), and collecting optimizer statistics, benefit from temporary tablespaces. The performance gains are a

significant with Real Application Clusters. Sort Segments One or more temporary tablespaces can be used only for sort segments. A temporary tablespace is not the same as a tablespace that a user designates for temporary segments, which can be any tablespace available to the user. No permanent schema objects can reside in a temporary tablespace. Sort segments are used when a segment is shared by multiple sort operations. One sort segment exists for every instance that performs a sort operation in a given tablespace. Temporary tablespaces provide performance improvements when you have multiple sorts that are too large to fit into memory. The sort segment of a given temporary tablespace is created at the time of the first sort operation. The sort segment expands by allocating extents until the segment size is equal to or greater than the total storage demands of all of the active sorts running on that instance. Creation of Temporary Tablespaces Create temporary tablespaces by using the CREATE TABLESPACE or CREATE TEMPORARY TABLESPACE statement. Transport of Tablespaces Between Databases A transportable tablespace lets you move a subset of an Oracle database from one Oracle database to another, even across different platforms. You can clone a tablespace and plug it into another database, copying the tablespace between databases, or you can unplug a tablespace from one Oracle database and plug it into another Oracle database, moving the tablespace between databases. Moving data by transporting tablespaces can be orders of magnitude faster than either export/import or unload/load of the same data, because transporting a tablespace involves only copying datafiles and integrating the tablespace metadata. When you transport tablespaces you can also move index data, so you do not have to rebuild the indexes after importing or loading the table data. You can transport tablespaces across platforms. (Many, but not all, platforms are supported for cross-platform tablespace transport.) This can be used for the following:

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* Provide an easier and more efficient means for content providers to publish structured data and distribute it to customers running Oracle on a different platform * Simplify the distribution of data from a data warehouse environment to data marts which are often running on smaller platforms * Enable the sharing of read only tablespaces across a heterogeneous cluster * Allow a database to be migrated from one platform to another Tablespace Repository A tablespace repository is a collection of tablespace sets. Tablespace repositories are built on file group repositories, but tablespace repositories only contain the files required to move or copy tablespaces between databases. Different tablespace sets may be stored in a tablespace repository, and different versions of a particular tablespace set also may be stored. A version of a tablespace set in a tablespace repository consists of the following files: * The Data Pump export dump file for the tablespace set * The Data Pump log file for the export * The datafiles that comprise the tablespace set

How to Move or Copy a Tablespace to Another Database To move or copy a set of tablespaces, you must make the tablespaces read only, copy the datafiles of these tablespaces, and use export/import to move the database information (metadata) stored in the data dictionary. Both the datafiles and the metadata export file must be copied to the target database. The transport of these files can be done using any facility for copying flat files, such as the operating system copying facility, ftp, or publishing on CDs. After copying the datafiles and importing the metadata, you can optionally put the tablespaces in read/write mode. The first time a tablespace's datafiles are opened under Oracle Database with the COMPATIBLE initialization parameter set to 10 or higher, each file identifies the platform to which it belongs. These files have identical on disk formats for file header blocks, which are used for file identification and verification. Read only and offline files get the compatibility advanced after they are made read/write or are brought online. This implies that tablespaces that are read only prior to Oracle Database 10g must be made read/write at least once before they can a

use the cross platform transportable feature. Note: In a database with a locally managed SYSTEM tablespace, dictionary tablespaces cannot be created. It is possible to plug in a dictionary managed tablespace using the transportable feature, but it cannot be made writable. Overview of Datafiles A tablespace in an Oracle database consists of one or more physical datafiles. A datafile can be associated with only one tablespace and only one database. Oracle creates a datafile for a tablespace by allocating the specified amount of disk space plus the overhead required for the file header. When a datafile is created, the operating system under which Oracle runs is responsible for clearing old information and authorizations from a file before allocating it to Oracle. If the file is large, this process can take a significant amount of time. The first tablespace in any database is always the SYSTEM tablespace, so Oracle automatically allocates the first datafiles of any database for the SYSTEM tablespace during database creation. See Also: Your Oracle operating system-specific documentation for information about the amount of space required for the file header of datafiles on your operating system Datafile Contents When a datafile is first created, the allocated disk space is formatted but does not contain any user data. However, Oracle reserves the space to hold the data for future segments of the associated tablespace—it is used exclusively by Oracle. As the data grows in a tablespace, Oracle uses the free space in the associated datafiles to allocate extents for the segment. The data associated with schema objects in a tablespace is physically stored in one or more of the datafiles that constitute the tablespace. Note that a schema object does not correspond to a specific datafile; rather, a datafile is a repository for the data of any schema object within a specific tablespace. Oracle allocates space for the data associated with a schema object in one or more datafiles of a a

tablespace. Therefore, a schema object can span one or more datafiles. Unless table striping is used (where data is spread across more than one disk), the database administrator and end users cannot control which datafile stores a schema object. Size of Datafiles You can alter the size of a datafile after its creation or you can specify that a datafile should dynamically grow as schema objects in the tablespace grow. This functionality enables you to have fewer datafiles for each tablespace and can simplify administration of datafiles. Note: You need sufficient space on the operating system for expansion. Offline Datafiles You can take tablespaces offline or bring them online at any time, except for the SYSTEM tablespace. All of the datafiles of a tablespace are taken offline or brought online as a unit when you take the tablespace offline or bring it online, respectively. You can take individual datafiles offline. However, this is usually done only during some database recovery procedures. Temporary Datafiles Locally managed temporary tablespaces have temporary datafiles (tempfiles), which are similar to ordinary datafiles, with the following exceptions: * Tempfiles are always set to NOLOGGING mode. * You cannot make a tempfile read only. * You cannot create a tempfile with the ALTER DATABASE statement. * Media recovery does not recognize tempfiles: o BACKUP CONTROLFILE does not generate any information for tempfiles. o CREATE CONTROLFILE cannot specify any information about tempfiles. * When you create or resize tempfiles, they are not always guaranteed allocation of disk space for the file size specified. On certain file systems (for example, UNIX) disk blocks are allocated not at file creation or resizing, but before the blocks are accessed. Caution: a

This enables fast tempfile creation and resizing; however, the disk could run of space later when the tempfiles are accessed. * Tempfile information is shown in the dictionary view DBA_TEMP_FILES and the dynamic performance view V$TEMPFILE, but not in DBA_DATA_FILES or the V$DATAFILE view. Overview of Control Files The database control file is a small binary file necessary for the database to start and operate successfully. A control file is updated continuously by Oracle during database use, so it must be available for writing whenever the database is open. If for some reason the control file is not accessible, then the database cannot function properly. Each control file is associated with only one Oracle database. Control File Contents A control file contains information about the associated database that is required for access by an instance, both at startup and during normal operation. Control file information can be modified only by Oracle; no database administrator or user can edit a control file. Among other things, a control file contains information such as: * * * * * * * * * * * *

The database name The timestamp of database creation The names and locations of associated datafiles and redo log files Tablespace information Datafile offline ranges The log history Archived log information Backup set and backup piece information Backup datafile and redo log information Datafile copy information The current log sequence number Checkpoint information

The database name and timestamp originate at database creation. The database name is taken from either the name specified by the DB_NAME initialization parameter or the name used in the CREATE DATABASE statement. Each time that a datafile or a redo log file is added to, renamed in, or a

dropped from the database, the control file is updated to reflect this physical structure change. These changes are recorded so that: * Oracle can identify the datafiles and redo log files to open during database startup * Oracle can identify files that are required or available in case database recovery is necessary Therefore, if you make a change to the physical structure of your database (using ALTER DATABASE statements), then you should immediately make a backup of your control file. Control files also record information about checkpoints. Every three seconds, the checkpoint process (CKPT) records information in the control file about the checkpoint position in the redo log. This information is used during database recovery to tell Oracle that all redo entries recorded before this point in the redo log group are not necessary for database recovery; they were already written to the datafiles. 19.CONTROLFILES-Oracle DBA What Is a Control File? Every Oracle database has a control file. A control file is a small binary file that records the physical structure of the database and includes: * The database name * Names and locations of associated datafiles and online redo log files * The timestamp of the database creation * The current log sequence number * Checkpoint information The control file must be available for writing by the Oracle database server whenever the database is open. Without the control file, the database cannot be mounted and recovery is difficult. The control file of an Oracle database is created at the same time as the database. By default, at least one copy of the control file is created during database creation. On some operating systems the default is to create multiple copies. You should create two or more copies of the control file during database creation. You might also need to create a

control files later, if you lose control files or want to change particular settings in the control files. Guidelines for Control Files This section describes guidelines you can use to manage the control files for a database, and contains the following topics: * Provide Filenames for the Control Files * Multiplex Control Files on Different Disks * Place Control Files Appropriately * Back Up Control Files * Manage the Size of Control Files Provide Filenames for the Control Files You specify control file names using the CONTROL_FILES initialization parameter in the database's initialization parameter file (see "Creating Initial Control Files"). The instance startup procedure recognizes and opens all the listed files. The instance writes to and maintains all listed control files during database operation. If you do not specify files for CONTROL_FILES before database creation, and you are not using the Oracle Managed Files feature, Oracle creates a control file and uses a default filename. The default name is operating system specific. Multiplex Control Files on Different Disks Every Oracle database should have at least two control files, each stored on a different disk. If a control file is damaged due to a disk failure, the associated instance must be shut down. Once the disk drive is repaired, the damaged control file can be restored using the intact copy of the control file from the other disk and the instance can be restarted. In this case, no media recovery is required. The following describes the behavior of multiplexed control files: * Oracle writes to all filenames listed for the initialization parameter CONTROL_FILES in the database's initialization parameter file.

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* The first file listed in the CONTROL_FILES parameter is the only file read by the Oracle database server during database operation. * If any of the control files become unavailable during database operation, the instance becomes inoperable and should be aborted. Note: Oracle strongly recommends that your database has a minimum of two control files and that they are located on separate disks. Place Control Files Appropriately As already suggested, each copy of a control file should be stored on a different disk drive. One practice is to store a control file copy on every disk drive that stores members of online redo log groups, if the online redo log is multiplexed. By storing control files in these locations, you minimize the risk that all control files and all groups of the online redo log will be lost in a single disk failure. Back Up Control Files It is very important that you back up your control files. This is true initially, and at any time after you change the physical structure of your database. Such structural changes include: * Adding, dropping, or renaming datafiles * Adding or dropping a tablespace, or altering the read-write state of the tablespace * Adding or dropping redo log files or groups The methods for backing up control files are discussed in "Backing Up Control Files". Manage the Size of Control Files The main determinants of a control file's size are the values set for the MAXDATAFILES, MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, and MAXINSTANCES parameters in the CREATE DATABASE statement that created the associated database. Increasing the values of these parameters increases the size of a control file of the associated database. Creating Control Files a

This section describes ways to create control files, and contains the following topics: * Creating Initial Control Files * Creating Additional Copies, Renaming, and Relocating Control Files * Creating New Control Files Creating Initial Control Files The initial control files of an Oracle database are created when you issue the CREATE DATABASE statement. The names of the control files are specified by the CONTROL_FILES parameter in the initialization parameter file used during database creation. The filenames specified in CONTROL_FILES should be fully specified and are operating system specific. The following is an example of a CONTROL_FILES initialization parameter: CONTROL_FILES = (/u01/oracle/prod/control01.ctl, /u02/oracle/prod/control02.ctl, /u03/oracle/prod/control03.ctl) If files with the specified names currently exist at the time of database creation, you must specify the CONTROLFILE REUSE clause in the CREATE DATABASE statement, or else an error occurs. Also, if the size of the old control file differs from the SIZE parameter of the new one, you cannot use the REUSE option. The size of the control file changes between some releases of Oracle, as well as when the number of files specified in the control file changes. Configuration parameters such as MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES, and MAXINSTANCES affect control file size. You can subsequently change the value of the CONTROL_FILES initialization parameter to add more control files or to change the names or locations of existing control files. Creating Additional Copies, Renaming, and Relocating Control Files You can create an an additional control file copy by copying an existing control file to a new location and adding the file's name to the list of control files. Similarly, you rename an existing control file by copying a

the file to its new name or location, and changing the file's name in the control file list. In both cases, to guarantee that control files do not change during the procedure, shut down the instance before copying the control file. To Multiplex or Move Additional Copies of the Current Control Files 1. Shut down the database. 2. Copy an existing control file to a different location, using operating system commands. 3. Edit the CONTROL_FILES parameter in the database's initialization parameter file to add the new control file's name, or to change the existing control filename. 4. Restart the database. Creating New Control Files This section discusses when and how to create new control files. When to Create New Control Files It is necessary for you to create new control files in the following situations: * All control files for the database have been permanently damaged and you do not have a control file backup. * You want to change one of the permanent database parameter settings originally specified in the CREATE DATABASE statement. These settings include the database's name and the following parameters: MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES, and MAXINSTANCES. For example, you would change a database's name if it conflicted with another database's name in a distributed environment. Or, as another example, you can change the value of MAXLOGFILES if the original setting is too low. The CREATE CONTROLFILE Statement You can create a new control file for a database using the CREATE CONTROLFILE statement. The following statement creates a new

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control file for the prod database (formerly a database that used a different database name): CREATE CONTROLFILE SET DATABASE prod LOGFILE GROUP 1 ('/u01/oracle/prod/redo01_01.log', '/u01/oracle/prod/redo01_02.log'), GROUP 2 ('/u01/oracle/prod/redo02_01.log', '/u01/oracle/prod/redo02_02.log'), GROUP 3 ('/u01/oracle/prod/redo03_01.log', '/u01/oracle/prod/redo03_02.log') NORESETLOGS DATAFILE '/u01/oracle/prod/system01.dbf' SIZE 3M, '/u01/oracle/prod/rbs01.dbs' SIZE 5M, '/u01/oracle/prod/users01.dbs' SIZE 5M, '/u01/oracle/prod/temp01.dbs' SIZE 5M MAXLOGFILES 50 MAXLOGMEMBERS 3 MAXLOGHISTORY 400 MAXDATAFILES 200 MAXINSTANCES 6 ARCHIVELOG; Cautions: * The CREATE CONTROLFILE statement can potentially damage specified datafiles and online redo log files. Omitting a filename can cause loss of the data in that file, or loss of access to the entire a

database. Employ caution when using this statement and be sure to follow the instructions in "Steps for Creating New Control Files". * If the database had forced logging enabled before creating the new control file, and you want it to continue to be enabled, then you must specify the FORCE LOGGING clause in the CREATE CONTROLFILE statement. See "Specifying FORCE LOGGING Mode".

Steps for Creating New Control Files Complete the following steps to create a new control file. 1. Make a list of all datafiles and online redo log files of the database. If you follow recommendations for control file backups as discussed in "Backing Up Control Files" , you will already have a list of datafiles and online redo log files that reflect the current structure of the database. However, if you have no such list, executing the following statements will produce one. SELECT MEMBER FROM V$LOGFILE; SELECT NAME FROM V$DATAFILE; SELECT VALUE FROM V$PARAMETER WHERE NAME = 'CONTROL_FILES'; If you have no such lists and your control file has been damaged so that the database cannot be opened, try to locate all of the datafiles and online redo log files that constitute the database. Any files not specified in Step 5 are not recoverable once a new control file has been created. Moreover, if you omit any of the files that make up the SYSTEM tablespace, you might not be able to recover the database. 2. Shut down the database. If the database is open, shut down the database normally if possible. Use the IMMEDIATE or ABORT options only as a last resort. 3. Back up all datafiles and online redo log files of the database. 4. Start up a new instance, but do not mount or open the database: STARTUP NOMOUNT

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5. Create a new control file for the database using the CREATE CONTROLFILE statement. When creating a new control file, select the RESETLOGS option if you have lost any online redo log groups in addition to control files. In this case, you will need to recover from the loss of the redo logs (Step 8). You must also specify the RESETLOGS option if you have renamed the database. Otherwise, select the NORESETLOGS option. 6. Store a backup of the new control file on an offline storage device. See "Backing Up Control Files" for instructions for creating a backup. 7. Edit the CONTROL_FILES initialization parameter for the database to indicate all of the control files now part of your database as created in Step 5 (not including the backup control file). If you are renaming the database, edit the DB_NAME parameter to specify the new name. 8. Recover the database if necessary. If you are not recovering the database, skip to Step 9. If you are creating the control file as part of recovery, recover the database. If the new control file was created using the NORESETLOGS option (Step 5), you can recover the database with complete, closed database recovery. If the new control file was created using the RESETLOGS option, you must specify USING BACKUP CONTROL FILE. If you have lost online or archived redo logs or datafiles, use the procedures for recovering those files. 9. Open the database using one of the following methods: * If you did not perform recovery, or you performed complete, closed database recovery in Step 8, open the database normally. ALTER DATABASE OPEN; * If you specified RESETLOGS when creating the control file, use the ALTER DATABASE statement, indicating RESETLOGS. ALTER DATABASE OPEN RESETLOGS; The database is now open and available for use. Troubleshooting After Creating Control Files

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After issuing the CREATE CONTROLFILE statement, you may encounter some common errors. This section describes the most common control file usage errors, and contains the following topics: * Checking for Missing or Extra Files * Handling Errors During CREATE CONTROLFILE Checking for Missing or Extra Files After creating a new control file and using it to open the database, check the alert file to see if Oracle has detected inconsistencies between the data dictionary and the control file, such as a datafile that the data dictionary includes but the control file does not list. If a datafile exists in the data dictionary but not in the new control file, Oracle creates a placeholder entry in the control file under the name MISSINGnnnn (where nnnn is the file number in decimal). MISSINGnnnn is flagged in the control file as being offline and requiring media recovery. The actual datafile corresponding to MISSINGnnnn can be made accessible by renaming MISSINGnnnn so that it points to the datafile only if the datafile was read-only or offline normal. If, on the other hand, MISSINGnnnn corresponds to a datafile that was not read-only or offline normal, then the rename operation cannot be used to make the datafile accessible, because the datafile requires media recovery that is precluded by the results of RESETLOGS. In this case, you must drop the tablespace containing the datafile. In contrast, if a datafile indicated in the control file is not present in the data dictionary, Oracle removes references to it from the new control file. In both cases, Oracle includes an explanatory message in the alert.log file to let you know what was found. Handling Errors During CREATE CONTROLFILE If Oracle sends you an error (usually error ORA-01173, ORA-01176, ORA-01177, ORA-01215, or ORA-01216) when you attempt to mount and open the database after creating a new control file, the most likely cause is that you omitted a file from the CREATE CONTROLFILE statement or included one that should not have been listed. In this case, you should restore the files you backed up in Step 3 and repeat the procedure from Step 4, using the correct filenames. Backing Up Control Files a

Use the ALTER DATABASE BACKUP CONTROLFILE statement to back up your control files. You have two options: 1. Back up the control file to a binary file (duplicate of existing control file) using the following statement: ALTER DATABASE BACKUP CONTROLFILE TO '/oracle/backup/control.bkp'; 2. Produce SQL statements that can later be used to re-create your control file: ALTER DATABASE BACKUP CONTROLFILE TO TRACE; This command writes a SQL script to the database's trace file where it can be captured and edited to reproduce the control file.

Recovering a Control File Using a Current Copy This section presents ways that you can recover your control file from a current backup or from a multiplexed copy. Recovering from Control File Corruption Using a Control File Copy This procedure assumes that one of the control files specified in the CONTROL_FILES parameter is corrupted, the control file directory is still accessible, and you have a multiplexed copy of the control file. 1. With the instance shut down, use an operating system command to overwrite the bad control file with a good copy: % cp /u01/oracle/prod/control03.ctl /u01/oracle/prod/control02.ctl 2. Start SQL*Plus and open the database: SQL> STARTUP Recovering from Permanent Media Failure Using a Control File Copy This procedure assumes that one of the control files specified in the CONTROL_FILES parameter is inaccessible due to a permanent media failure, and you have a multiplexed copy of the control file.

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1. With the instance shut down, use an operating system command to copy the current copy of the control file to a new, accessible location: % cp /u01/oracle/prod/control01.ctl /u04/oracle/prod/control03.ctl 2. Edit the CONTROL_FILES parameter in the initialization parameter file to replace the bad location with the new location: CONTROL_FILES = (/u01/oracle/prod/control01.ctl, /u02/oracle/prod/control02.ctl, /u04/oracle/prod/control03.ctl) 3. Start SQL*Plus and open the database: SQL> STARTUP In any case where you have multiplexed control files, and you must get the database up in minimum time, you can do so by editing the CONTROL_FILES initialization parameter to remove the bad control file and restarting the database immediately. Then you can perform the reconstruction of the bad control file and at some later time shut down and restart the database after editing the CONTROL_FILES initialization parameter to include the recovered control file. Dropping Control Files You can drop control files from the database. For example, you might want to do so if the location of a control file is no longer appropriate. Remember that the database must have at least two control files at all times. 1. Shut down the database. 2. Edit the CONTROL_FILES parameter in the database's initialization parameter file to delete the old control file's name. 3. Restart the database. Note: This operation does not physically delete the unwanted control file from the disk. Use operating system commands to delete the unnecessary file after you have dropped the control file from the database. a

Displaying Control File Information The following views display information about control files: View Description V$DATABASE Displays database information from the control file V$CONTROLFILE Lists the names of control files V$CONTROLFILE_RECORD_SECTION Displays information about control file record sections V$PARAMETER Can be used to display the names of control files as specified in the CONTROL_FILES initialization parameter This example lists the names of the control files. SQL> SELECT NAME FROM V$CONTROLFILE; NAME ------------------------------------/u01/oracle/prod/control01.ctl /u02/oracle/prod/control02.ctl /u03/oracle/prod/control03.ctl 2.OracleDBA-Interview Questions-1 1)What are the contents of redolog files Ans) redolog files contains redo entries contain ddl, 2)How are redolog files significant to the database Ans)for recovery purpose 3)How are redlogs organized Ans)in the form of redolog groups 4)Create a database dbredo with following specs a

1)DMT System tablespace of 100M 2)LMT Users tablespace of 50M 3)Multiplex controlfiles (2) - OMF 4)One Loggroup multiplex (4) - OMF - 512k size of the logfile 5)Limit the No. of groups to 2 6)Limit the No. of Members to 3 7)Limit the No. of datafiles 5 8)Instance recovery should not take more than 5 minutes 9)Database in archivelog mode 5)What is the alternate name for identical log files Ans)Log members 6)How is the Numbering of the members in a log group done Ans) 1a 1b 1c 2a 2b 2c 3a 3b 3c 7)How is the sizing done for the members of a log group Ans)>alter database 8)What is an online Log Member 9)How do you verify what is the current Log sequence No. Ans) archive log list 10)How are the redologs used 11)What happens when the pointer from one Log file to another file 12)When are the contents of the logfile written 13) In a database of 4 log groups, after theh 4th log group is full, what is the next step Ans) If the db in archivelog mode then the first log group redo entries archived after that writes on the first redo log group If the db in non archive mode then it over writes the first log group 14)How and where are the dirty buffers written 15)What determines the dirty buffers to be written Ans) 1) for every checkpoint occuring 2) It reaches threshold value 3) there is no free buffers in the DBC 16)What is a checkpoint. Under what situations does a checkpoint occur Ans)it is a point in log files up to which every thing is gaurenteed to be a

writen to the data filles. and it is the point from where 17)Where is the checkpoint information recorded Ans) 1) controlfile 2) datafile header 18)How can be checkpoint forced Ans) Alter system checkpoint; 20.Redo Log File Management What Is the Online Redo Log? The most crucial structure for recovery operations is the online redo log, which consists of two or more preallocated files that store all changes made to the database as they occur. Every instance of an Oracle database has an associated online redo log to protect the database in case of an instance failure. Redo Threads Each database instance has its own online redo log groups. These online redo log groups, multiplexed or not, are called an instance's thread of online redo. In typical configurations, only one database instance accesses an Oracle database, so only one thread is present. When running Oracle Real Application Clusters, however, two or more instances concurrently access a single database and each instance has its own thread. This chapter describes how to configure and manage the online redo log when the Oracle9i Real Application Clusters feature is not used. Hence, the thread number can be assumed to be 1 in all discussions and examples of statements. Online Redo Log Contents Online redo log files are filled with redo records. A redo record, also called a redo entry, is made up of a group of change vectors, each of which is a description of a change made to a single block in the database. For example, if you change a salary value in an employee table, you generate a redo record containing change vectors that describe changes to the data segment block for the table, the rollback segment data block, and the transaction table of the rollback segments. Redo entries record data that you can use to reconstruct all changes made to the database, including the rollback segments. Therefore, the a

online redo log also protects rollback data. When you recover the database using redo data, Oracle reads the change vectors in the redo records and applies the changes to the relevant blocks. Redo records are buffered in a circular fashion in the redo log buffer of the SGA (see "How Oracle Writes to the Online Redo Log") and are written to one of the online redo log files by the Oracle background process Log Writer (LGWR). Whenever a transaction is committed, LGWR writes the transaction's redo records from the redo log buffer of the SGA to an online redo log file, and a system change number (SCN) is assigned to identify the redo records for each committed transaction. Only when all redo records associated with a given transaction are safely on disk in the online logs is the user process notified that the transaction has been committed. Redo records can also be written to an online redo log file before the corresponding transaction is committed. If the redo log buffer fills, or another transaction commits, LGWR flushes all of the redo log entries in the redo log buffer to an online redo log file, even though some redo records may not be committed. If necessary, Oracle can roll back these changes. How Oracle Writes to the Online Redo Log The online redo log of a database consists of two or more online redo log files. Oracle requires a minimum of two files to guarantee that one is always available for writing while the other is being archived (if in ARCHIVELOG mode). LGWR writes to online redo log files in a circular fashion. When the current online redo log file fills, LGWR begins writing to the next available online redo log file. When the last available online redo log file is filled, LGWR returns to the first online redo log file and writes to it, starting the cycle again. Figure 7-1 illustrates the circular writing of the online redo log file. The numbers next to each line indicate the sequence in which LGWR writes to each online redo log file. Filled online redo log files are available to LGWR for reuse depending on whether archiving is enabled. * If archiving is disabled (NOARCHIVELOG mode), a filled online redo log file is available once the changes recorded in it have been written to the datafiles.

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* If archiving is enabled (ARCHIVELOG mode), a filled online redo log file is available to LGWR once the changes recorded in it have been written to the datafiles and once the file has been archived. Figure Circular Use of Online Redo Log Files by LGWR

Active (Current) and Inactive Online Redo Log Files At any given time, Oracle uses only one of the online redo log files to store redo records written from the redo log buffer. The online redo log file that LGWR is actively writing to is called the current online redo log file. Online redo log files that are required for instance recovery are called active online redo log files. Online redo log files that are not required for instance recovery are called inactive. If you have enabled archiving (ARCHIVELOG mode), Oracle cannot reuse or overwrite an active online log file until ARCn has archived its contents. If archiving is disabled (NOARCHIVELOG mode), when the last online redo log file fills writing continues by overwriting the first available active file. Log Switches and Log Sequence Numbers A log switch is the point at which Oracle ends writing to one online redo log file and begins writing to another. Normally, a log switch occurs when the current online redo log file is completely filled and writing must continue to the next online redo log file. However, you can specify that a log switch occurs in a time-based manner, a

regardless of whether the current online redo log file is completely filled. You can also force log switches manually. Oracle assigns each online redo log file a new log sequence number every time that a log switch occurs and LGWR begins writing to it. If Oracle archives online redo log files, the archived log retains its log sequence number. The online redo log file that is cycled back for use is given the next available log sequence number. Each online or archived redo log file is uniquely identified by its log sequence number. During crash, instance, or media recovery, Oracle properly applies redo log files in ascending order by using the log sequence number of necessary archived and online redo log files. Planning the Online Redo Log This section describes guidelines you should consider when configuring a database instance's online redo log, and contains the following topics: * Multiplexing Online Redo Log Files * Placing Online Redo Log Members on Different Disks * Setting the Size of Online Redo Log Members * Choosing the Number of Online Redo Log Files * Controlling Archive Lag Multiplexing Online Redo Log Files

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Oracle provides the capability to multiplex an instance's online redo log files to safeguard against damage to its online redo log files. When multiplexing online redo log files, LGWR concurrently writes the same redo log information to multiple identical online redo log files, thereby eliminating a single point of redo log failure. Note: Oracle recommends that you multiplex your redo log files. The loss of the log file data can be catastrophic if recovery is required. Figure Multiplexed Online Redo Log Files The corresponding online redo log files are called groups. Each online redo log file in a group is called a member. In Figure , A_LOG1 and B_LOG1 are both members of Group 1, A_LOG2 and B_LOG2 are both members of Group 2, and so forth. Each member in a group must be exactly the same size. Notice that each member of a group is concurrently active, or, concurrently written to by LGWR, as indicated by the identical log sequence numbers assigned by LGWR. In Figure 7-2, first LGWR writes to A_LOG1 in conjunction with B_LOG1, then A_LOG2 in conjunction with B_LOG2, and so on. LGWR never writes concurrently to members of different groups (for example, to A_LOG1 and B_LOG2). Responding to Online Redo Log Failure Whenever LGWR cannot write to a member of a group, Oracle marks that member as INVALID and writes an error message to the LGWR trace file and to the database's alert file to indicate the problem with the inaccessible files. LGWR reacts differently when certain online redo log members are unavailable, depending on the reason for the unavailability. If Then LGWR can successfully write to at least one member in a group Writing proceeds as normal. LGWR simply writes to the available members of a group and ignores the unavailable members. LGWR cannot access the next group at a log switch because the group needs to be archived

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Database operation temporarily halts until the group becomes available, or, until the group is archived. All members of the next group are inaccessible to LGWR at a log switch because of media failure Oracle returns an error and the database instance shuts down. In this case, you may need to perform media recovery on the database from the loss of an online redo log file. If the database checkpoint has moved beyond the lost redo log, media recovery is not necessary since Oracle has saved the data recorded in the redo log to the datafiles. Simply drop the inaccessible redo log group. If Oracle did not archive the bad log, use ALTER DATABASE CLEAR UNARCHIVED LOG to disable archiving before the log can be dropped. If all members of a group suddenly become inaccessible to LGWR while it is writing to them Oracle returns an error and the database instance immediately shuts down. In this case, you may need to perform media recovery. If the media containing the log is not actually lost--for example, if the drive for the log was inadvertently turned off--media recovery may not be needed. In this case, you only need to turn the drive back on and let Oracle perform instance recovery. Legal and Illegal Configurations To safeguard against a single point of online redo log failure, a multiplexed online redo log is ideally symmetrical: all groups of the online redo log have the same number of members. Nevertheless, Oracle does not require that a multiplexed online redo log be symmetrical. For example, one group can have only one member, while other groups have two members. This configuration protects against disk failures that temporarily affect some online redo log members but leave others intact. The only requirement for an instance's online redo log is that it have at least two groups. Figure legal and illegal multiplexed online redo log configurations. The second configuration is illegal because it has only one group. Figure Legal and Illegal Multiplexed Online Redo Log Configuration

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Placing Online Redo Log Members on Different Disks When setting up a multiplexed online redo log, place members of a group on different disks. If a single disk fails, then only one member of a group becomes unavailable to LGWR and other members remain accessible to LGWR, so the instance can continue to function. If you archive the redo log, spread online redo log members across disks to eliminate contention between the LGWR and ARCn background processes. For example, if you have two groups of duplexed online redo log members, place each member on a different disk and set your archiving destination to a fifth disk. Consequently, there is never contention between LGWR (writing to the members) and ARCn (reading the members). Datafiles and online redo log files should also be on different disks to reduce contention in writing data blocks and redo records. Setting the Size of Online Redo Log Members When setting the size of online redo log files, consider whether you will be archiving the redo log. Online redo log files should be sized so that a filled group can be archived to a single unit of offline storage media (such as a tape or disk), with the least amount of space on the medium left unused. For example, suppose only one filled online redo log group can fit on a tape and 49% of the tape's storage capacity remains a

unused. In this case, it is better to decrease the size of the online redo log files slightly, so that two log groups could be archived for each tape. With multiplexed groups of online redo logs, all members of the same group must be the same size. Members of different groups can have different sizes. However, there is no advantage in varying file size between groups. If checkpoints are not set to occur between log switches, make all groups the same size to guarantee that checkpoints occur at regular intervals. Choosing the Number of Online Redo Log Files The best way to determine the appropriate number of online redo log files for a database instance is to test different configurations. The optimum configuration has the fewest groups possible without hampering LGWR's writing redo log information. In some cases, a database instance may require only two groups. In other situations, a database instance may require additional groups to guarantee that a recycled group is always available to LGWR. During testing, the easiest way to determine if the current online redo log configuration is satisfactory is to examine the contents of the LGWR trace file and the database's alert log. If messages indicate that LGWR frequently has to wait for a group because a checkpoint has not completed or a group has not been archived, add groups. Consider the parameters that can limit the number of online redo log files before setting up or altering the configuration of an instance's online redo log. The following parameters limit the number of online redo log files that you can add to a database: * The MAXLOGFILES parameter used in the CREATE DATABASE statement determines the maximum number of groups of online redo log files for each database. Group values can range from 1 to MAXLOGFILES. The only way to override this upper limit is to re-create the database or its control file. Thus, it is important to consider this limit before creating a database. If MAXLOGFILES is not specified for the CREATE DATABASE statement, Oracle uses an operating system specific default value. * The MAXLOGMEMBERS parameter used in the CREATE DATABASE statement determines the maximum number of members for each group. As with MAXLOGFILES, the only way to override this upper limit is to re-create the database or control file. Thus, it is important to consider this limit before creating a database. If no MAXLOGMEMBERS a

parameter is specified for the CREATE DATABASE statement, Oracle uses an operating system default value. You can force all enabled online redo log threads to switch their current logs in a time-based fashion. In a primary/standby configuration, changes are made available to the standby database by archiving and shipping logs of the primary site to the standby database. The changes that are being applied by the standby database can lag the changes that are occurring on the primary database. This lag can happen because the standby database must wait for the changes in the primary database's online redo log to be archived (into the archived redo log) and then shipped to it. To control or limit this lag, you set the ARCHIVE_LAG_TARGET initialization parameter. Setting this parameter allows you to limit, measured in time, how long the lag can become. Setting the ARCHIVE_LAG_TARGET Initialization Parameter When you set the ARCHIVE_LAG_TARGET initialization parameter, you cause Oracle to examine an instance's current online redo log periodically. If the following conditions are met the instance will switch the log: * The current log was created prior to n seconds ago, and the estimated archival time for the current log is m seconds (proportional to the number of redo blocks used in the current log), where n + m exceeds the value of the ARCHIVE_LAG_TARGET initialization parameter. * The current log contains redo records. In an Oracle Real Application Clusters environment, the instance also nudges other threads into switching and archiving logs if they are falling behind. This can be particularly useful when one instance in the cluster is more idle than the other instances (as when you are running a 2-node primary/secondary configuration of Oracle Real Application Clusters). Initialization parameter ARCHIVE_LAG_TARGET specifies the target of how many seconds of redo the standby could lose in the event of a primary shutdown or crash if the Data Guard environment is not configured in a no-data-loss mode. It also provides an upper limit of how long (in the number of seconds) the current log of the primary database can span. Because the estimated archival time is also considered, this is not the exact log switch time. a

The following initialization parameter setting sets the log switch interval to 30 minutes (a typical value). ARCHIVE_LAG_TARGET = 1800 A value of 0 disables this time-based log switching functionality. This is the default setting. You can set the ARCHIVE_LAG_TARGET initialization parameter even if there is no standby database. For example, the ARCHIVE_LAG_TARGET parameter can be set specifically to force logs to be switched and archived. ARCHIVE_LAG_TARGET is a dynamic parameter and can be set with the ALTER SYSTEM SET statement. Caution: The ARCHIVE_LAG_TARGET parameter must be set to the same value in all instances of an Oracle Real Application Clusters environment. Failing to do so results in unspecified behavior and is strongly discouraged. Factors Affecting the Setting of ARCHIVE_LAG_TARGET Consider the following factors when determining if you want to set the ARCHIVE_LAG_TARGET parameter and in determining the value for this parameter. * Overhead of switching (as well as archiving) logs * How frequently normal log switches occur as a result of log full conditions * How much redo loss is tolerated in the standby database Setting ARCHIVE_LAG_TARGET may not be very useful if natural log switches already occur more frequently than the interval specified. However, in the case of irregularities of redo generation speed, the interval does provide an upper limit for the time range each current log covers. If the ARCHIVE_LAG_TARGET initialization parameter is set to a very low value, there can be a negative impact on performance. This can force frequent log switches. Set the parameter to a reasonable value so as not to degrade the performance of the primary database.

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Creating Online Redo Log Groups and Members Plan the online redo log of a database and create all required groups and members of online redo log files during database creation. However, there are situations where you might want to create additional groups or members. For example, adding groups to an online redo log can correct redo log group availability problems. To create new online redo log groups and members, you must have the ALTER DATABASE system privilege. A database can have up to MAXLOGFILES groups. To create a new group of online redo log files, use the SQL statement ALTER DATABASE with the ADD LOGFILE clause. The following statement adds a new group of redo logs to the database: ALTER DATABASE ADD LOGFILE ('/oracle/dbs/log1c.rdo', '/oracle/dbs/log2c.rdo') SIZE 500K; Note: Use fully specify filenames of new log members to indicate where the operating system file should be created. Otherwise, the files will be created in either the default or current directory of the database server, depending upon your operating system. You can also specify the number that identifies the group using the GROUP option: ALTER DATABASE ADD LOGFILE GROUP 10 ('/oracle/dbs/log1c.rdo', '/oracle/dbs/log2c.rdo') SIZE 500K; Using group numbers can make administering redo log groups easier. However, the group number must be between 1 and MAXLOGFILES. Do not skip redo log file group numbers (that is, do not number your groups 10, 20, 30, and so on), or you will consume space in the control files of the database.

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Creating Online Redo Log Members In some cases, it might not be necessary to create a complete group of online redo log files. A group could already exist, but not be complete because one or more members of the group were dropped (for example, because of a disk failure). In this case, you can add new members to an existing group. To create new online redo log members for an existing group, use the SQL statement ALTER DATABASE with the ADD LOG MEMBER parameter. The following statement adds a new redo log member to redo log group number 2: ALTER DATABASE ADD LOGFILE MEMBER '/oracle/dbs/log2b.rdo' TO GROUP 2; Notice that filenames must be specified, but sizes need not be. The size of the new members is determined from the size of the existing members of the group. When using the ALTER DATABASE statement, you can alternatively identify the target group by specifying all of the other members of the group in the TO parameter, as shown in the following example: ALTER DATABASE ADD LOGFILE MEMBER '/oracle/dbs/log2c.rdo' TO ('/oracle/dbs/log2a.rdo', '/oracle/dbs/log2b.rdo'); Note: Fully specify the filenames of new log members to indicate where the operating system files should be created. Otherwise, the files will be created in either the default or current directory of the database server, depending upon your operating system. You may also note that the status of the new log member is shown as INVALID. This is normal and it will change to active (blank) when it is first used. Relocating and Renaming Online Redo Log Members You can use operating system commands to relocate online redo logs, then use the ALTER DATABASE statement to make their new names (locations) known to the database. This procedure is necessary, for example, if the disk currently used for some online redo log files is going to be removed, or if datafiles and a number of online redo log files are stored on the same disk and should be separated to reduce contention. a

To rename online redo log members, you must have the ALTER DATABASE system privilege. Additionally, you might also need operating system privileges to copy files to the desired location and privileges to open and back up the database. Before relocating your redo logs, or making any other structural changes to the database, completely back up the database in case you experience problems while performing the operation. As a precaution, after renaming or relocating a set of online redo log files, immediately back up the database's control file. Use the following steps for relocating redo logs. The example used to illustrate these steps assumes: * The log files are located on two disks: diska and diskb. * The online redo log is duplexed: one group consists of the members /diska/logs/log1a.rdo and /diskb/logs/log1b.rdo, and the second group consists of the members /diska/logs/log2a.rdo and /diskb/logs/log2b.rdo. * The online redo log files located on diska must be relocated to diskc. The new filenames will reflect the new location: /diskc/logs/log1c.rdo and /diskc/logs/log2c.rdo. Steps for Renaming Online Redo Log Members 1. Shut down the database. SHUTDOWN 2. Copy the online redo log files to the new location. Operating system files, such as online redo log members, must be copied using the appropriate operating system commands. See your operating system specific documentation for more information about copying files. Note: You can execute an operating system command to copy a file (or perform other operating system commands) without exiting SQL*Plus by using the HOST command. Some operating systems allow you to use a character in place of the word HOST. For example, you can use ! in UNIX.

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The following example uses operating system commands (UNIX) to move the online redo log members to a new location: mv /diska/logs/log1a.rdo /diskc/logs/log1c.rdo mv /diska/logs/log2a.rdo /diskc/logs/log2c.rdo 3. Startup the database, mount, but do not open it. CONNECT / as SYSDBA STARTUP MOUNT 4. Rename the online redo log members. Use the ALTER DATABASE statement with the RENAME FILE clause to rename the database's online redo log files. ALTER DATABASE RENAME FILE '/diska/logs/log1a.rdo', '/diska/logs/log2a.rdo' TO '/diskc/logs/log1c.rdo', '/diskc/logs/log2c.rdo'; 5. Open the database for normal operation. The online redo log alterations take effect when the database is opened. ALTER DATABASE OPEN; Dropping Online Redo Log Groups and Members In some cases, you may want to drop an entire group of online redo log members. For example, you want to reduce the number of groups in an instance's online redo log. In a different case, you may want to drop one or more specific online redo log members. For example, if a disk failure occurs, you may need to drop all the online redo log files on the failed disk so that Oracle does not try to write to the inaccessible files. In other situations, particular online redo log files become unnecessary. For example, a file might be stored in an inappropriate location. Dropping Log Groups

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To drop an online redo log group, you must have the ALTER DATABASE system privilege. Before dropping an online redo log group, consider the following restrictions and precautions: * An instance requires at least two groups of online redo log files, regardless of the number of members in the groups. (A group is one or more members.) * You can drop an online redo log group only if it is inactive. If you need to drop the current group, first force a log switch to occur. * Make sure an online redo log group is archived (if archiving is enabled) before dropping it. To see whether this has happened, use the V$LOG view. SELECT GROUP#, ARCHIVED, STATUS FROM V$LOG; GROUP# ARC STATUS --------- --- ---------------1 YES ACTIVE 2 NO CURRENT 3 YES INACTIVE 4 YES INACTIVE Drop an online redo log group with the SQL statement ALTER DATABASE with the DROP LOGFILE clause. The following statement drops redo log group number 3: ALTER DATABASE DROP LOGFILE GROUP 3; When an online redo log group is dropped from the database, and you are not using the Oracle Managed Files feature, the operating system files are not deleted from disk. Rather, the control files of the associated database are updated to drop the members of the group from the database structure. After dropping an online redo log group, make sure that the drop completed successfully, and then use the appropriate operating system command to delete the dropped online redo log files.

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When using Oracle-managed files, the cleanup of operating systems files is done automatically for you. Dropping Online Redo Log Members To drop an online redo log member, you must have the ALTER DATABASE system privilege. Consider the following restrictions and precautions before dropping individual online redo log members: * It is permissible to drop online redo log files so that a multiplexed online redo log becomes temporarily asymmetric. For example, if you use duplexed groups of online redo log files, you can drop one member of one group, even though all other groups have two members each. However, you should rectify this situation immediately so that all groups have at least two members, and thereby eliminate the single point of failure possible for the online redo log. * An instance always requires at least two valid groups of online redo log files, regardless of the number of members in the groups. (A group is one or more members.) If the member you want to drop is the last valid member of the group, you cannot drop the member until the other members become valid. To see a redo log file's status, use the V$LOGFILE view. A redo log file becomes INVALID if Oracle cannot access it. It becomes STALE if Oracle suspects that it is not complete or correct. A stale log file becomes valid again the next time its group is made the active group. * You can drop an online redo log member only if it is not part of an active or current group. If you want to drop a member of an active group, first force a log switch to occur. * Make sure the group to which an online redo log member belongs is archived (if archiving is enabled) before dropping the member. To see whether this has happened, use the V$LOG view. To drop specific inactive online redo log members, use the ALTER DATABASE statement with the DROP LOGFILE MEMBER clause. The following statement drops the redo log /oracle/dbs/log3c.rdo: ALTER DATABASE DROP LOGFILE MEMBER '/oracle/dbs/log3c.rdo'; When an online redo log member is dropped from the database, the operating system file is not deleted from disk. Rather, the control files of the associated database are updated to drop the member from the database structure. After dropping an online redo log file, make sure a

that the drop completed successfully, and then use the appropriate operating system command to delete the dropped online redo log file. To drop a member of an active group, you must first force a log switch. Forcing Log Switches A log switch occurs when LGWR stops writing to one online redo log group and starts writing to another. By default, a log switch occurs automatically when the current online redo log file group fills. You can force a log switch to make the currently active group inactive and available for online redo log maintenance operations. For example, you want to drop the currently active group, but are not able to do so until the group is inactive. You may also wish to force a log switch if the currently active group needs to be archived at a specific time before the members of the group are completely filled. This option is useful in configurations with large online redo log files that take a long time to fill. To force a log switch, you must have the ALTER SYSTEM privilege. Use the ALTER SYSTEM statement with the SWITCH LOGFILE clause. The following statement forces a log switch: ALTER SYSTEM SWITCH LOGFILE; Verifying Blocks in Redo Log Files You can configure Oracle to use checksums to verify blocks in the redo log files. If you set the initialization parameter DB_BLOCK_CHECKSUM to TRUE, block checking is enabled for all Oracle database blocks written to disk, including redo log blocks. The default value of DB_BLOCK_CHECKSUM is FALSE. If you enable block checking, Oracle computes a checksum for each redo log block written to the current log. Oracle writes the checksum in the header of the block. Oracle uses the checksum to detect corruption in a redo log block. Oracle tries to verify the redo log block when it writes the block to an archive log file and when the block is read from an archived log during recovery. If Oracle detects a corruption in a redo log block while trying to archive it, the system attempts to read the block from another member in the group. If the block is corrupted in all members the redo log group, then archiving cannot proceed. a

Note: There is some overhead and decrease in database performance with DB_BLOCK_CHECKSUM enabled. Monitor your database performance to decide if the benefit of using data block checksums to detect corruption outweights the performance impact. Clearing an Online Redo Log File An online redo log file might become corrupted while the database is open, and ultimately stop database activity because archiving cannot continue. In this situation the ALTER DATABASE CLEAR LOGFILE statement can be used reinitialize the file without shutting down the database. The following statement clears the log files in redo log group number 3: ALTER DATABASE CLEAR LOGFILE GROUP 3; This statement overcomes two situations where dropping redo logs is not possible: * If there are only two log groups * The corrupt redo log file belongs to the current group If the corrupt redo log file has not been archived, use the UNARCHIVED keyword in the statement. ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP 3; This statement clears the corrupted redo logs and avoids archiving them. The cleared redo logs are available for use even though they were not archived. If you clear a log file that is needed for recovery of a backup, then you can no longer recover from that backup. Oracle writes a message in the alert log describing the backups from which you cannot recover. Note: If you clear an unarchived redo log file, you should make another backup of the database.

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If you want to clear an unarchived redo log that is needed to bring an offline tablespace online, use the UNRECOVERABLE DATAFILE clause in the ALTER DATABASE CLEAR LOGFILE statement. If you clear a redo log needed to bring an offline tablespace online, you will not be able to bring the tablespace online again. You will have to drop the tablespace or perform an incomplete recovery. Note that tablespaces taken offline normal do not require recovery. Viewing Online Redo Log Information Use the following views to display online redo log information. View Description V$LOG Displays the redo log file information from the control file V$LOGFILE Identifies redo log groups and members and member status V$LOG_HISTORY Contains log history information The following query returns the control file information about the online redo log for a database. SELECT * FROM V$LOG; GROUP# THREAD# SEQ BYTES MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIM ------ ------- ----- ------- ------- --- --------- ------------- --------1 1 10605 1048576 1 YES ACTIVE 11515628 16-APR-00 2 1 10606 1048576 1 NO CURRENT 11517595 16-APR-00 3 1 10603 1048576 1 YES INACTIVE 11511666 16-APR-00 4 1 10604 1048576 1 YES INACTIVE 11513647 16-APR-00

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To see the names of all of the member of a group, use a query similar to the following: SELECT * FROM V$LOGFILE; GROUP# STATUS MEMBER ------ ------- ---------------------------------1 D:\ORANT\ORADATA\IDDB2\REDO04.LOG 2 D:\ORANT\ORADATA\IDDB2\REDO03.LOG 3 D:\ORANT\ORADATA\IDDB2\REDO02.LOG 4 D:\ORANT\ORADATA\IDDB2\REDO01.LOG 21.Undo Tablespace Management Managing the Undo Tablespace This chapter describes how to manage the undo tablespace, which stores information used to roll back changes to the Oracle Database. It contains the following topics: What Is Undo? Every Oracle Database must have a method of maintaining information that is used to roll back, or undo, changes to the database. Such information consists of records of the actions of transactions, primarily before they are committed. These records are collectively referred to as undo. Undo records are used to: Roll back transactions when a ROLLBACK statement is issued Recover the database Provide read consistency Analyze data as of an earlier point in time by using Oracle Flashback Query Recover from logical corruptions using Oracle Flashback features

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When a ROLLBACK statement is issued, undo records are used to undo changes that were made to the database by the uncommitted transaction. During database recovery, undo records are used to undo any uncommitted changes applied from the redo log to the datafiles. Undo records provide read consistency by maintaining the before image of the data for users who are accessing the data at the same time that another user is changing it. Introduction to Automatic Undo Management This section introduces the concepts of Automatic Undo Management and discusses the following topics: Overview of Automatic Undo Management Oracle provides a fully automated mechanism, referred to as automatic undo management, for managing undo information and space. In this management mode, you create an undo tablespace, and the server automatically manages undo segments and space among the various active sessions. You set the UNDO_MANAGEMENT initialization parameter to AUTO to enable automatic undo management. A default undo tablespace is then created at database creation. An undo tablespace can also be created explicitly. When the instance starts, the database automatically selects the first available undo tablespace. If no undo tablespace is available, then the instance starts without an undo tablespace and stores undo records in the SYSTEM tablespace. This is not recommended in normal circumstances, and an alert message is written to the alert log file to warn that the system is running without an undo tablespace. If the database contains multiple undo tablespaces, you can optionally specify at startup that you want to use a specific undo tablespace. This is done by setting the UNDO_TABLESPACE initialization parameter, as shown in this example: UNDO_TABLESPACE = undotbs_01 In this case, if you have not already created the undo tablespace (in this example, undotbs_01), the STARTUP command fails. The UNDO_TABLESPACE parameter can be used to assign a specific undo tablespace to an instance in an Oracle Real Application Clusters environment.

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The following is a summary of the initialization parameters for automatic undo management:

Initialization Parameter

Description

UNDO_MANAGEMENT If AUTO, use automatic undo management. The default is MANUAL. UNDO_TABLESPACE

An optional dynamic parameter specifying the name of an undo tablespace. This parameter should be used only when the database has multiple undo tablespaces and you want to direct the database instance to use a particular undo tablespace.

When automatic undo management is enabled, if the initialization parameter file contains parameters relating to manual undo management, they are ignored. Undo Retention After a transaction is committed, undo data is no longer needed for rollback or transaction recovery purposes. However, for consistent read purposes, long-running queries may require this old undo information for producing older images of data blocks. Furthermore, the success of several Oracle Flashback features can also depend upon the availability of older undo information. For these reasons, it is desirable to retain the old undo information for as long as possible. When automatic undo management is enabled, there is always a current undo retention period, which is the minimum amount of time that Oracle Database attempts to retain old undo information before overwriting it. Old (committed) undo information that is older than the current undo retention period is said to be expired. Old undo information with an age that is less than the current undo retention period is said to be unexpired. Oracle Database automatically tunes the undo retention period based on undo tablespace size and system activity. You can specify a minimum undo retention period (in seconds) by setting the UNDO_RETENTION initialization parameter. The database makes its best effort to honor the specified minimum undo retention period,

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provided that the undo tablespace has space available for new transactions. When available space for new transactions becomes short, the database begins to overwrite expired undo. If the undo tablespace has no space for new transactions after all expired undo is overwritten, the database may begin overwriting unexpired undo information. If any of this overwritten undo information is required for consistent read in a current long-running query, the query could fail with the snapshot too old error message. The following points explain the exact impact of the UNDO_RETENTION parameter on undo retention: The UNDO_RETENTION parameter is ignored for a fixed size undo tablespace. The database may overwrite unexpired undo information when tablespace space becomes low. For an undo tablespace with the AUTOEXTEND option enabled, the database attempts to honor the minimum retention period specified by UNDO_RETENTION. When space is low, instead of overwriting unexpired undo information, the tablespace auto-extends. If the MAXSIZE clause is specified for an auto-extending undo tablespace, when the maximum size is reached, the database may begin to overwrite unexpired undo information. Retention Guarantee To guarantee the success of long-running queries or Oracle Flashback operations, you can enable retention guarantee. If retention guarantee is enabled, the specified minimum undo retention is guaranteed; the database never overwrites unexpired undo data even if it means that transactions fail due to lack of space in the undo tablespace. If retention guarantee is not enabled, the database can overwrite unexpired undo when space is low, thus lowering the undo retention for the system. This option is disabled by default. WARNING: Enabling retention guarantee can cause multiple DML operations to fail. Use with caution. You enable retention guarantee by specifying the RETENTION GUARANTEE clause for the undo tablespace when you create it with either the CREATE DATABASE or CREATE UNDO TABLESPACE statement. Or, you can later specify this clause in an ALTER TABLESPACE statement. You disable retention guarantee with the RETENTION NOGUARANTEE clause. a

You can use the DBA_TABLESPACES view to determine the retention guarantee setting for the undo tablespace. A column named RETENTION contains a value of GUARANTEE, NOGUARANTEE, or NOT APPLY (used for tablespaces other than the undo tablespace). Automatic Tuning of Undo Retention Oracle Database automatically tunes the undo retention period based on how the undo tablespace is configured. If the undo tablespace is fixed size, the database tunes the retention period for the best possible undo retention for that tablespace size and the current system load. This tuned retention period can be significantly greater than the specified minimum retention period. If the undo tablespace is configured with the AUTOEXTEND option, the database tunes the undo retention period to be somewhat longer than the longest-running query on the system at that time. Again, this tuned retention period can be greater than the specified minimum retention period. Note: Automatic tuning of undo retention is not supported for LOBs. This is because undo information for LOBs is stored in the segment itself and not in the undo tablespace. For LOBs, the database attempts to honor the minimum undo retention period specified by UNDO_RETENTION. However, if space becomes low, unexpired LOB undo information may be overwritten. You can determine the current retention period by querying the TUNED_UNDORETENTION column of the V$UNDOSTAT view. This view contains one row for each 10-minute statistics collection interval over the last 4 days. (Beyond 4 days, the data is available in the DBA_HIST_UNDOSTAT view.) TUNED_UNDORETENTION is given in seconds. select to_char(begin_time, 'DD-MON-RR HH24:MI') begin_time, to_char(end_time, 'DD-MON-RR HH24:MI') end_time, tuned_undoretention from v$undostat order by end_time; BEGIN_TIME END_TIME TUNED_UNDORETENTION

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--------------- --------------- ------------------04-FEB-05 00:01 04-FEB-05 00:11 12100 ... 07-FEB-05 23:21 07-FEB-05 23:31 86700 07-FEB-05 23:31 07-FEB-05 23:41 86700 07-FEB-05 23:41 07-FEB-05 23:51 86700 07-FEB-05 23:51 07-FEB-05 23:52 86700 576 rows selected. . Undo Retention Tuning and Alert Thresholds For a fixed size undo tablespace, the database calculates the maximum undo retention period based on database statistics and on the size of the undo tablespace. For optimal undo management, rather than tuning based on 100% of the tablespace size, the database tunes the undo retention period based on 85% of the tablespace size, or on the warning alert threshold percentage for space used, whichever is lower. (The warning alert threshold defaults to 85%, but can be changed.) Therefore, if you set the warning alert threshold of the undo tablespace below 85%, this may reduce the tuned length of the undo retention period. Setting the Undo Retention Period You set the undo retention period by setting the UNDO_RETENTION initialization parameter. This parameter specifies the desired minimum undo retention period in seconds. the current undo retention period may be automatically tuned to be greater than UNDO_RETENTION, or, unless retention guarantee is enabled, less than UNDO_RETENTION if space is low. To set the undo retention period: Do one of the following: Set UNDO_RETENTION in the initialization parameter file. UNDO_RETENTION = 1800

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Change UNDO_RETENTION at any time using the ALTER SYSTEM statement: ALTER SYSTEM SET UNDO_RETENTION = 2400;

The effect of an UNDO_RETENTION parameter change is immediate, but it can only be honored if the current undo tablespace has enough space. Sizing the Undo Tablespace You can size the undo tablespace appropriately either by using automatic extension of the undo tablespace or by using the Undo Advisor for a fixed sized tablespace. Using Auto-Extensible Tablespaces Oracle Database supports automatic extension of the undo tablespace to facilitate capacity planning of the undo tablespace in the production environment. When the system is first running in the production environment, you may be unsure of the space requirements of the undo tablespace. In this case, you can enable automatic extension of the undo tablespace so that it automatically increases in size when more space is needed. You do so by including the AUTOEXTEND keyword when you create the undo tablespace. Sizing Fixed-Size Undo Tablespaces If you have decided on a fixed-size undo tablespace, the Undo Advisor can help you estimate needed capacity. You can access the Undo Advisor through Enterprise Manager or through the DBMS_ADVISOR PL/SQL package. Enterprise Manager is the preferred method of accessing the advisor. The Undo Advisor relies for its analysis on data collected in the Automatic Workload Repository (AWR). It is therefore important that the AWR have adequate workload statistics available so that the Undo Advisor can make accurate recommendations. For newly created databases, adequate statistics may not be available immediately. In such cases, an auto-extensible undo tablespace can be used.

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An adjustment to the collection interval and retention period for AWR statistics can affect the precision and the type of recommendations that the advisor produces. To use the Undo Advisor, you first estimate these two values: The length of your expected longest running query After the database has been up for a while, you can view the Longest Running Query field on the Undo Management page of Enterprise Manager. The longest interval that you will require for flashback operations For example, if you expect to run Flashback Queries for up to 48 hours in the past, your flashback requirement is 48 hours. You then take the maximum of these two undo retention values and use that value to look up the required undo tablespace size on the Undo Advisor graph. The Undo Advisor PL/SQL Interface You can activate the Undo Advisor by creating an undo advisor task through the advisor framework. The following example creates an undo advisor task to evaluate the undo tablespace. The name of the advisor is 'Undo Advisor'. The analysis is based on Automatic Workload Repository snapshots, which you must specify by setting parameters START_SNAPSHOT and END_SNAPSHOT. In the following example, the START_SNAPSHOT is "1" and END_SNAPSHOT is "2". DECLARE tid NUMBER; tname VARCHAR2(30); oid NUMBER; BEGIN DBMS_ADVISOR.CREATE_TASK('Undo Advisor', tid, tname, 'Undo Advisor Task'); DBMS_ADVISOR.CREATE_OBJECT(tname, 'UNDO_TBS', null, null, null, 'null', oid); a

DBMS_ADVISOR.SET_TASK_PARAMETER(tname, 'TARGET_OBJECTS', oid); DBMS_ADVISOR.SET_TASK_PARAMETER(tname, 'START_SNAPSHOT', 1); DBMS_ADVISOR.SET_TASK_PARAMETER(tname, 'END_SNAPSHOT', 2); DBMS_ADVISOR.SET_TASK_PARAMETER(name, 'INSTANCE', 1); DBMS_ADVISOR.execute_task(tname); end; / After you have created the advisor task, you can view the output and recommendations in the Automatic Database Diagnostic Monitor in Enterprise Manager. This information is also available in the DBA_ADVISOR_* data dictionary views. Managing Undo Tablespaces This section describes the various steps involved in undo tablespace management and contains the following sections: Creating an Undo Tablespace There are two methods of creating an undo tablespace. The first method creates the undo tablespace when the CREATE DATABASE statement is issued. This occurs when you are creating a new database, and the instance is started in automatic undo management mode (UNDO_MANAGEMENT = AUTO). The second method is used with an existing database. It uses the CREATE UNDO TABLESPACE statement. You cannot create database objects in an undo tablespace. It is reserved for system-managed undo data. Oracle Database enables you to create a single-file undo tablespace. Single-file, or bigfile, tablespaces are discussed in "Bigfile Tablespaces". Using CREATE DATABASE to Create an Undo Tablespace You can create a specific undo tablespace using the UNDO TABLESPACE clause of the CREATE DATABASE statement. a

The following statement illustrates using the UNDO TABLESPACE clause in a CREATE DATABASE statement. The undo tablespace is named undotbs_01 and one datafile, /u01/oracle/rbdb1/undo0101.dbf, is allocated for it. CREATE DATABASE rbdb1 CONTROLFILE REUSE . . . UNDO TABLESPACE undotbs_01 DATAFILE '/u01/oracle/rbdb1/undo0101.dbf'; If the undo tablespace cannot be created successfully during CREATE DATABASE, the entire CREATE DATABASE operation fails. You must clean up the database files, correct the error and retry the CREATE DATABASE operation. The CREATE DATABASE statement also lets you create a single-file undo tablespace at database creation. Using the CREATE UNDO TABLESPACE Statement The CREATE UNDO TABLESPACE statement is the same as the CREATE TABLESPACE statement, but the UNDO keyword is specified. The database determines most of the attributes of the undo tablespace, but you can specify the DATAFILE clause. This example creates the undotbs_02 undo tablespace with the AUTOEXTEND option: CREATE UNDO TABLESPACE undotbs_02 DATAFILE '/u01/oracle/rbdb1/undo0201.dbf' SIZE 2M REUSE AUTOEXTEND ON; You can create more than one undo tablespace, but only one of them can be active at any one time. Altering an Undo Tablespace

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Undo tablespaces are altered using the ALTER TABLESPACE statement. However, since most aspects of undo tablespaces are system managed, you need only be concerned with the following actions: Adding a datafile Renaming a datafile Bringing a datafile online or taking it offline Beginning or ending an open backup on a datafile Enabling and disabling undo retention guarantee These are also the only attributes you are permitted to alter. If an undo tablespace runs out of space, or you want to prevent it from doing so, you can add more files to it or resize existing datafiles. The following example adds another datafile to undo tablespace undotbs_01: ALTER TABLESPACE undotbs_01 ADD DATAFILE '/u01/oracle/rbdb1/undo0102.dbf' AUTOEXTEND ON NEXT 1M MAXSIZE UNLIMITED; You can use the ALTER DATABASE...DATAFILE statement to resize or extend a datafile. Dropping an Undo Tablespace Use the DROP TABLESPACE statement to drop an undo tablespace. The following example drops the undo tablespace undotbs_01: DROP TABLESPACE undotbs_01; An undo tablespace can only be dropped if it is not currently used by any instance. If the undo tablespace contains any outstanding transactions (for example, a transaction died but has not yet been recovered), the DROP TABLESPACE statement fails. However, since DROP TABLESPACE drops an undo tablespace even if it contains unexpired undo information (within retention period), you must be

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careful not to drop an undo tablespace if undo information is needed by some existing queries. DROP TABLESPACE for undo tablespaces behaves like DROP TABLESPACE...INCLUDING CONTENTS. All contents of the undo tablespace are removed. Switching Undo Tablespaces You can switch from using one undo tablespace to another. Because the UNDO_TABLESPACE initialization parameter is a dynamic parameter, the ALTER SYSTEM SET statement can be used to assign a new undo tablespace. The following statement switches to a new undo tablespace: ALTER SYSTEM SET UNDO_TABLESPACE = undotbs_02; Assuming undotbs_01 is the current undo tablespace, after this command successfully executes, the instance uses undotbs_02 in place of undotbs_01 as its undo tablespace. If any of the following conditions exist for the tablespace being switched to, an error is reported and no switching occurs: The tablespace does not exist The tablespace is not an undo tablespace The tablespace is already being used by another instance (in a RAC environment only) The database is online while the switch operation is performed, and user transactions can be executed while this command is being executed. When the switch operation completes successfully, all transactions started after the switch operation began are assigned to transaction tables in the new undo tablespace. The switch operation does not wait for transactions in the old undo tablespace to commit. If there are any pending transactions in the old undo tablespace, the old undo tablespace enters into a PENDING OFFLINE mode (status). In this mode, existing transactions can continue to execute, but undo records for new user transactions cannot be stored in this undo tablespace.

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An undo tablespace can exist in this PENDING OFFLINE mode, even after the switch operation completes successfully. A PENDING OFFLINE undo tablespace cannot be used by another instance, nor can it be dropped. Eventually, after all active transactions have committed, the undo tablespace automatically goes from the PENDING OFFLINE mode to the OFFLINE mode. From then on, the undo tablespace is available for other instances (in an Oracle Real Application Cluster environment). If the parameter value for UNDO TABLESPACE is set to '' (two single quotes), then the current undo tablespace is switched out and the next available undo tablespace is switched in. Use this statement with care because there may be no undo tablespace available. The following example unassigns the current undo tablespace: ALTER SYSTEM SET UNDO_TABLESPACE = ''; Establishing User Quotas for Undo Space The Oracle Database Resource Manager can be used to establish user quotas for undo space. The Database Resource Manager directive UNDO_POOL allows DBAs to limit the amount of undo space consumed by a group of users (resource consumer group). You can specify an undo pool for each consumer group. An undo pool controls the amount of total undo that can be generated by a consumer group. When the total undo generated by a consumer group exceeds its undo limit, the current UPDATE transaction generating the undo is terminated. No other members of the consumer group can perform further updates until undo space is freed from the pool. When no UNDO_POOL directive is explicitly defined, users are allowed unlimited undo space. Migrating to Automatic Undo Management If you are currently using rollback segments to manage undo space, Oracle strongly recommends that you migrate your database to automatic undo management. Oracle Database provides a function that provides information on how to size your new undo tablespace based on the configuration and usage of the rollback segments in your system. DBA privileges are required to execute this function: DECLARE utbsiz_in_MB NUMBER; a

BEGIN utbsiz_in_MB := DBMS_UNDO_ADV.RBU_MIGRATION; end; / The function returns the sizing information directly. Viewing Information About Undo This section lists views that are useful for viewing information about undo space in the automatic undo management mode and provides some examples. In addition to views listed here, you can obtain information from the views available for viewing tablespace and datafile information. Oracle Database also provides proactive help in managing tablespace disk space use by alerting you when tablespaces run low on available space. In addition to the proactive undo space alerts, Oracle Database also provides alerts if your system has long-running queries that cause SNAPSHOT TOO OLD errors. To prevent excessive alerts, the long query alert is issued at most once every 24 hours. When the alert is generated, you can check the Undo Advisor Page of Enterprise Manager to get more information about the undo tablespace. The following dynamic performance views are useful for obtaining space information about the undo tablespace:

View

Description

V$UNDOSTAT

Contains statistics for monitoring and tuning undo space. Use this view to help estimate the amount of undo space required for the current workload. The database also uses this information to help tune undo usage in the system. This view is meaningful only in automatic undo management mode.

V$ROLLSTAT

For automatic undo management mode, information reflects behavior of the undo segments in the undo tablespace

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V$TRANSACTION

Contains undo segment information

DBA_UNDO_EXTENTS Shows the status and size of each extent in the undo tablespace. DBA_HIST_UNDOSTAT Contains statistical snapshots of V$UNDOSTAT information.

The V$UNDOSTAT view is useful for monitoring the effects of transaction execution on undo space in the current instance. Statistics are available for undo space consumption, transaction concurrency, the tuning of undo retention, and the length and SQL ID of longrunning queries in the instance. Each row in the view contains statistics collected in the instance for a ten-minute interval. The rows are in descending order by the BEGIN_TIME column value. Each row belongs to the time interval marked by (BEGIN_TIME, END_TIME). Each column represents the data collected for the particular statistic in that time interval. The first row of the view contains statistics for the (partial) current time period. The view contains a total of 576 rows, spanning a 4 day cycle. The following example shows the results of a query on the V$UNDOSTAT view. SELECT TO_CHAR(BEGIN_TIME, 'MM/DD/YYYY HH24:MI:SS') BEGIN_TIME, TO_CHAR(END_TIME, 'MM/DD/YYYY HH24:MI:SS') END_TIME, UNDOTSN, UNDOBLKS, TXNCOUNT, MAXCONCURRENCY AS "MAXCON" FROM v$UNDOSTAT WHERE rownum