DBA

Rollback Segments are renamed Undo logs.Traditionally transaction undo information stroed in rollback segments until a commit or rollback statment was issued.

Automatic undo management allows the dba to how long undo information should retained after commit.preventing 'snapshot-too old' error on long running queries.

This is done by undo_retention parameter.the default is 900 seconds(5minutes).you can set this parameter to gurantte that oracle keeps undo logs for extended period of time.

rather then having to define rollback segments,you can simply define an undo tablespace and oracle let take the rest and automatic tuning. Undo_management=AUTO

sql> Select sum(a.bytes) "Undo_Size" from v$datafile a, v$tablespace b,dba_tablespaces c where c.contents='UNDO' and c.status='ONLINE' and b.name=c.tablespace_name and a.ts#=b.ts#;

sql>select max(undoblks/((end_time-begin_time)*3600*24)) " Undo_block_per_second" from v$undostat;

sql>select to_number(value) from 'db_block_size' from v$parameter where name='db_block_size';

**** 04/04/2008 Oracle 10g OCA 1z0-042 question

a.3 hours

b.6months

c.until last year
d.until last commit

e.until the point when the undo tablespace was refreshed.

f.until the database is shutdown and the memory erased.