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Don Burleson Blog 







Buffer cache size factors into Oracle optimizer execution plans

Oracle Database Tips by Donald BurlesonApril 19,  2015

Are the data buffer cache sizes considered when the Oracle cost-based optimizer chooses an optimal execution plan?

Larry Elkins published this test showing that the Oracle 9i cost-based optimizer considers the db_cache_size when costing-out execution plans for queries on production and a user acceptable test instance, with vastly different values for db_cache_size.  Here, Larry notes that the data buffer size appears to be considered in deriving the "best" execution plans for a query:

"I discovered PROD has a 5 gig buffer cache, UAT 5 MB.

The same query run against PROD and UAT to sanity check the UAT environment gave a different plan."

This is an important note for SQL optimization.  Oracle does not publish the internal machinations of their proprietary optimizer. 

Obviously, Oracle uses the values for the PGA for optimization:

  • pga_aggregate_target and/or hash_area_size - Determines whether to invoke has hash join vs. a nested loops join

  • pga_aggregate_target and/or sort_area_size - Determine whether to do a full-scan and a back-end sort vs. pulling the data pre-sorted vs. an index access.

Oracle notes that the self-set value for optimizer_index_caching helps the optimizer understand how much of an index is in the data buffer cache, but it now appears that the optimizer is intelligent enough to see when an instance has a "too small" data buffer (too small to cache the working set of frequently-referenced data).

In these cases, the optimizer should detect an "undersized" data buffer cache and choose an execution plan that optimizes the query for disk I/O, as-if none of the data was cached.

Remember, the Oracle CBO is a highly-guarded secret, and Oracle is not willing to expose their internals for fear of competition from lesser DBMS vendors.  For complete information on Oracle optimizer internals, see my book "Oracle Tuning: The Definitive Reference".

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