12c Regressed SQL - Unexplained Buffer Sort Used
(Doc ID 2176293.1)
Last updated on JULY 22, 2021
Applies to:Oracle Database - Enterprise Edition - Version 220.127.116.11 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 18.104.22.168, 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 22.214.171.124, the CTAS statement takes 7.5 minutes.
In 126.96.36.199, 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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