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Reversal of UNION ALL Branches May Degrades Performance if a Different Access Path is Chosen (Doc ID 1483223.1)

Last updated on OCTOBER 30, 2019

Applies to:

Oracle Database - Enterprise Edition - Version 11.2.0.2 and later
Oracle Database - Personal Edition - Version 11.2.0.2.0 and later
Oracle Database Backup Service - Version N/A and later
Oracle Database Cloud Exadata Service - Version N/A and later
Oracle Database Cloud Service - Version N/A and later
Information in this document applies to any platform.

Symptoms

Different execution plans and run-times are experienced for a query where the only change is to switch the sides that selects appear in a UNION ALL select. For example, the following select:

which has no effect on the results of the query but may choose a different access path and perform differently.

Changes

 Query text is changed so that the selects on either side of the UNION ALL Clause are switched.

Cause

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In this Document
Symptoms
Changes
Cause
Solution
References


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