Flashback and LOBs
Undo data for LOB columns in a table can take up gigabytes of disk space even for a single row;
therefore, to enable flashback operations for LOB columns, you must explicitly specify the retention
keyword in the storage clause for the LOB. This keyword is mutually exclusive with the pctversion
keyword, which specified a percentage of the table space for old versions of the LOBs. If you use
the retention keyword, old versions of a LOB are retained for the amount of time specified by the
UNDO_RETENTION parameter, just as any other table rows in the undo tablespace.
Migrating to Automatic Undo Management
To migrate your environment from manually managed rollback segments to Automatic Undo
Management, you need to know one thing: how large to size the undo tablespace based on the
usage of the rollback segments in manual undo mode. With all manual rollback segments online,
execute the procedure DBMS_UNDO_ADV.RBU_MIGRATION to return the size, in megabytes,
of the current rollback segment utilization:
SQL> variable undo_size number
SQL> begin
2 :undo_size := dbms_undo_adv.rbu_migration;
3 end;
4 /
240 Oracle Database 11g DBA Handbook
PL/SQL procedure successfully completed.
SQL> print :undo_size
UNDO_SIZE
----------
2840
SQL>
In this example, an undo tablespace created to replace the rollback segments should be at
least 2840MB, or 2.84GB, to support the undo requirements currently supported by rollback
segments.
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