Unexpected Poor Execution Plan When Non-Popular Value Missing from Histogram
(Doc ID 1563752.1)
Last updated on NOVEMBER 07, 2023
Applies to:
Oracle Database - Enterprise Edition - Version 11.2.0.1 to 11.2.0.4 [Release 11.2]Oracle Database Exadata Cloud Machine - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Oracle Database Cloud Exadata Service - Version N/A and later
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.
10053 trace will show:
Note: Buckets (Number of histograms) is less than NDV(Number of Distinct Values) for Frequency Histogram
Changes
Cause
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In this Document
| Symptoms |
| Changes |
| Cause |
| Solution |
| References |