Reversal of UNION ALL Branches May Degrades Performance if a Different Access Path is Chosen
Last updated on SEPTEMBER 15, 2016
Applies to:Oracle Server - Enterprise Edition - Version 22.214.171.124 and later
Oracle Server - Personal Edition - Version 126.96.36.199.0 and later
Oracle Server - Standard Edition - Version 188.8.131.52 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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