UNDO_RENTENTION

Please refer to this blog https://docs.oracle.com/cd/E11882_01/server.112/e25494/undo.htm#ADMIN11478 https://tamimdba.wordpress.com/tag/undo_retention/ http://www.akadia.com/services/ora_optimize_undo.html What Is Undo? Oracle Database creates and manages 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 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 data files. 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. Data Concurrency and Read Consistency ROLLBACK or UNDO is the backbone of the READ CONSISTENCY mechanism provided by Oracle. Multi-User Data Concurrency and Read Consistency mechanism make Oracle stand tall in Relational Database Management Systems (RDBMS) world. Starting in Oracle9i, rollback segments are re-named undo logs. Traditionally transaction undo information was stored in Rollback Segments until a commit or rollback statement was issued, at which point it was made available for overlaying. Best of all, automatic undo management allows the DBA to specify how long undo information should be retained after commit, preventing “snapshot too old” errors on long running queries. This is done by setting the UNDO_RETENTION parameter. The default is 900 seconds (5 minutes), and you can set this parameter to guarantee that Oracle keeps undo logs for extended periods of time. Rather than having to define and manage rollback segments, you can simply define an Undo tablespace and let Oracle take care of the rest. Turning on automatic undo management is easy. All you need to do is create an undo tablespace and set UNDO_MANAGEMENT = AUTO. UNDO_RETENTION is a parameter in the init.ora initialization parameters file that specifies the time period in seconds for which a system retains undo data for committed transactions. The flashback query can go upto the point of time specified as a value in the UNDO_RETENTION parameter. The properties of the UNDO_RETENTION parameter are mentioned below: Parameter type – Integer Default value – 900 Range of values – 0 to 232 – 1 Syntax – ALTER SYSTEM SET UNDO_RETENTION = < seconds >; However it is worth to tune the following important parameters 1. The size of the UNDO tablespace 2. The UNDO_RETENTION parameter Calculate UNDO_RETENTION for given UNDO Tabespace You can choose to allocate a specific size for the UNDO tablespace and then set the UNDO_RETENTION parameter to an optimal value according to the UNDO size and the database activity. If your disk space is limited and you do not want to allocate more space than necessary to the UNDO tablespace, this is the way to proceed. The following query will help you to optimize the UNDO_RETENTION parameter: OPTIMAL_UNDO_RETENTION = ACTUAL UNDO SIZE / (DB_BLOCK_SZIE * UNDO_BLOCK_PER_SEC) Because these following queries use the V$UNDOSTAT statistics, run the queries only after the database has been running with UNDO for a significant and representative time!