Suboptimal Plan For Skewed Data In Partitioned Table
Last updated on SEPTEMBER 16, 2016
Applies to:Oracle Database - Enterprise Edition - Version 188.8.131.52 and later
Information in this document applies to any platform.
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:
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