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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 AUGUST 04, 2018

Applies to:

Oracle Server - Enterprise Edition - Version and later
Oracle Server - Personal Edition - Version and later
Oracle Server - Standard Edition - Version and later
Information in this document applies to any platform.


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.


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


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