Parallel Processing

Performance feedback is one of the adaptive query optimizer features introduced in Oracle Database 12c. The aim of this short post isn’t to explain how it works, but to simply warn you about a buggy behavior in its configuration.

The parameters that control performance feedback are the following:

• OPTIMIZER_ADAPTIVE_FEATURES (12.1 only): if set to TRUE (default), performance feedback is enabled. Otherwise, it is disabled.
• OPTIMIZER_ADAPTIVE_STATISTICS (12.1 with patch 22652097, and 12.2): if set to TRUE, performance feedback is enabled. Otherwise, which is the default, it is disabled.
• PARALLEL_DEGREE_POLICY (12.1 and 12.2): it has to be set to ADAPTIVE (default is MANUAL) to enable performance feedback.

So far, so good.

From version 11.2 onward, the PARALLEL hint supports two syntaxes: object-level and statement-level. The object-level syntax, which is the only one available up to version 11.1, overrides the DOP associated to a tables. The statement-level syntax can not only override the PARALLEL_DEGREE_POLICY initialization parameter at the SQL statement level, but also force the utilization of parallel processing.

The statement-level PARALLEL hint supports the following values:

With the INMEMORY clause you can specify 4 sub-clauses:

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In the first and second post of this series I shared with you some basics about smart scan and gave some details about projection and restriction. The aim of this post is to cover the third basic technique: join filtering.
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The short answer to this question is “yes”, it does. Unfortunately, the distribution costs are not externalized through the execution plans and, as a result, this limitation (yes, it is really a limitation in the current implementation, not a bug) confuses everyone that carefully look at the information provided in an execution plan of a [...]