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Oracle sequence caching & performance

Oracle Database Tips by Donald Burleson

Question:  How to I cache an Oracle sequence for faster performance.  Can a sequence cache be used for tuning?

Answer:  Oracle has a method for caching frequently-referenced sequences, and you can also cache sequences with n-way Streams replication for fast access to sequence values.  Caching sequences is especially important for high-DML applications with lots on insert and update activity.

Caching an Oracle sequence

You can easily cache as sequence with the "add/alter sequence xxx cache" command.  The "cache" clause caches the specified number of sequence values into the buffers in the SGA. This speeds access, but all cached numbers are lost when the database is shut down. The default value is 20; maximum value is maxvalue-minvalue.

Delays from sequence cache enqueues

If there is insufficient caching of sequences, contention can result which will show up as an increase in service times for DML. If there are performance problems due to sequence cache waits, examine the row cache locks statistics in the v$system_event view to determine whether the problem is due to the use of Oracle sequences.

In RAC, sequence enqueue delays are shown in the eq_type column of the gv$enqueue_stat view.  A value of "SQ Enqueue" indicates that there is contention for sequences.

When creating sequences for a RAC environment, DBAs should use the NOORDER keyword to avoid an additional cause of SQ enqueue contention that is forced ordering of queued sequence values.

To reduce index contention for RAC environments you may also want to use the cache option and also ensure that you have the default value of noorder for your sequences.

Also,  see my notes on tuning to reduce index contention.

If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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