Reversal of UNION ALL Branches May Degrades Performance if a Different Access Path is Chosen

(Doc ID 1483223.1)

Last updated on SEPTEMBER 15, 2016

Applies to:

Oracle Server - Enterprise Edition - Version 11.2.0.2 and later
Oracle Server - Personal Edition - Version 11.2.0.2.0 and later
Oracle Server - Standard Edition - Version 11.2.0.2 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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