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

Last updated on NOVEMBER 02, 2016

Applies to:

Oracle Database - Enterprise Edition - Version 12.1.0.2 and later
Information in this document applies to any platform.

Symptoms

After migrating from 11g to 12.1.0.2, CTAS statement has a strange plan in 12c compared with 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 11.2.0.3, the CTAS statement takes 7.5 minutes.
In 12.1.0.2, the same CTAS takes 13 minutes.

 

Changes

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

 

Cause

Sign In with your My Oracle Support account

Don't have a My Oracle Support account? Click to get started

My Oracle Support provides customers with access to over a
Million Knowledge Articles and hundreds of Community platforms