Suboptimal Plan For Skewed Data In Partitioned Table (Doc ID 2181526.1)

Last updated on SEPTEMBER 16, 2016

Applies to:

Oracle Database - Enterprise Edition - Version 12.1.0.2 and later
Information in this document applies to any platform.

Symptoms

When the number of partitions in a table is 2*DoP or larger, then a PX PARTITION operation is selected instead of PX BLOCK operation.  The PX PARTITION operation does not handle skewness well in the data.  The extreme case is where a single partition contains all the data in which case the execution essentially runs serially with a single process reading all the data and the other slaves being idle.

Gathering statistics does not make any difference.

The following testcase demonstrates the issue:

 

Cause

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