Topic : Undo Tablespace

Differentiate Redo vs. Rollback vs. Undo?


Answer :

Redo: Every Oracle database has a set of (two or more) redo log files. The redo log records all changes made to data; including both uncommitted and committed changes. In addition to the online redo logs Oracle also stores archive redo logs. All redo logs are used in recovery situations.
Rollback: More specifically rollback segments. Rollback segments store the data as it was before changes were made. This is in contrast to the redo log which is a record of the insert/update/deletes.
Undo: Rollback segments. They both are really one in the same. Undo data is stored in the undo tablespace. Undo is helpful in building a read consistent view of data.

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Why it is not necessary to take UNDO backup?


Answer :

In fact it is not necessary to take UNDO tablespace backup either with COLD or HOT backup scripts but many of DBA include UNDO tablespace in their backup script. You know when you do some transactions; redo entries will be generated and accepted! Just like that other tablespace whenever any change happens to UNDO tablespace or UNDO segments oracle will generate redo entries. So even you not backed up the UNDO tablespace; you have the redo entries through which you can recover or rollback the transactions.

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What is the role of lock_sga parameter?


Answer :

The LOCK_SGA parameter; when set to TRUE; locks the entire SGA into physical memory. This parameter cannot be used with automatic memory management or automatic shared memory management.

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What is CSSCAN?


Answer :

CSSCAN (Database Character Set Scanner) is a SCAN tool that allows us to see the impact of a database character set change or assist us to correct an incorrect database nls_characterset setup. This helps us to determine the best approach for converting the database charecterset.

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Can we switch from using one undo tablespace to another.How?


Answer :

Yes;
SQL> ALTER SYSTEM SET UNDO_TABLESPACE = undotbs_02;

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