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12c Regressed SQL - Unexplained Buffer Sort Used (Doc ID 2176293.1)

Last updated on JULY 22, 2021

Applies to:

Oracle Database - Enterprise Edition - Version and later
Oracle Database Cloud Schema Service - Version N/A and later
Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Information in this document applies to any platform.


After migrating from 11g to, CTAS statement has a strange plan in 12c compared to 11g.

This is a CTAS that is executed using DEFAULT parallelism. Both the 11g and 12c databases are running on the same host with cpu_count set to 32 (default value).

in, the CTAS statement takes 7.5 minutes.
In, the same CTAS takes 13 minutes.



 What you see in 12c is now we have a “PX Deq: Table Q Ref” event. So, we have workers waiting a long time for something to happen.



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