Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 9 ■ REDO AND UNDO
ops$tkyte@ORA10G> alter index t_idx rebuild;
Index altered.
ops$tkyte@ORA10G> @mystat2
ops$tkyte@ORA10G> set echo off
NAME
V DIFF
---------- ---------- ----------------
redo size 15668084 62,292
and now it generates a mere 61KB of redo. But that index is “unprotected” now, if the data files
it was located in failed and had to be restored from a backup, we would lose that index data.
Understanding that fact is crucial. The index is not recoverable right now—we need a backup
to take place. Alternatively, the DBA could just re-create the index as we can re-create the
index directly from the table data as well.
NOLOGGING Wrap-Up
The operations that may be performed in a NOLOGGING mode are as follows:
• Index creations and ALTERs (rebuilds).
• Bulk INSERTs into a table using a “direct path insert” such as that available via the
/*+ APPEND */ hint or SQL*Loader direct path loads. The table data will not generate
redo, but all index modifications will (the indexes on this nonlogged table will
generate redo!).
• LOB operations (updates to large objects do not have to be logged).
• Table creations via CREATE TABLE AS SELECT.
• Various ALTER TABLE operations such as MOVE and SPLIT.
Used appropriately on an ARCHIVELOG mode database, NOLOGGING can speed up many
operations by dramatically reducing the amount of redo log generated. Suppose you have a
table you need to move from one tablespace to another. You can schedule this operation to
take place immediately before a backup occurs—you would ALTER the table to be NOLOGGING,
move it, rebuild the indexes (without logging as well), and then ALTER the table back to logging
mode. Now, an operation that might have taken X hours can happen in X/2 hours perhaps
(I’m not promising a 50 percent reduction in runtime!). The appropriate use of this feature
includes involvement of the DBA, or whoever is responsible for database backup and recovery
or any standby databases. If that person is not aware of the use of this feature, and a media
failure occurs, you may lose data, or the integrity of the standby database might be compromised.
This is something to seriously consider.