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 184.108.40.206 and later
Oracle Database - Personal Edition - Version 220.127.116.11.0 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.
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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