Unexpected Poor Execution Plan When Non-Popular Value Missing from Histogram (Doc ID 1563752.1)

Last updated on NOVEMBER 04, 2016

Applies to:

Oracle Database - Enterprise Edition - Version 11.2.0.1 to 11.2.0.4 [Release 11.2]
Information in this document applies to any platform.

Symptoms

A poor plan is chosen for predicates on certain columns.
For example, the majority of the time, the CBO chooses an INDEX UNIQUE SCAN on a primary key index, but for certain column values it will choose a less efficient INDEX RANGE SCAN on another index which contains the same leading columns contained within the primary key index.

SQLT report shows in Observation Section:

Frequency histogram with number of buckets not matching the number of distinct values.

 

Cause

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