Not sure why parallel hint didn't work correctly
Date: Tue, 16 Feb 2016 13:06:17 -0700
Message-ID: <CAJzM94ADenf3bDPwPiSoMwqbg7pa-FZ_0VVXw3m5KW+UBY_5Jw_at_mail.gmail.com>
Oracle EE 11.2.0.4
This is an Exadata with RAC database.
Database servers - 48 cores
I am just starting to learn about Exadata so I may be asking "silly" questions. User had a query with hint parallel(48) and looking only at a single partition with 1.6 billion rows in that partition, along with some joins to much smaller tables to get to the right set of rows. It was not returning results in a timely manner. The user indicated he let it run for over an hour without any results being returned so he killed it.
I did some testing and looked v$px_session, v$session_wait, got the execution plan, etc. to see what was going on. The 1.6 billion row table has a degree of 32. My test run without the hint completed in just over 12 minutes using parallel 32, 16 parallel sessions showing ACTIVE status on each node. v$session_wait showed 16 sessions on each node doing actual work.
When I put the hint back in, v$px_session showed 48 slaves, 24 on each of the nodes. All the sessions on one node showed status of INACTIVE, but the 24 on the other node were ACTIVE. v$session_wait showed only one session actively doing any work for this query.
No one on the team understands what is happening here. Why did the parallel(48) hint cause this behavior. Any insight would be greatly appreciated. I'm not sure if there is more information you need that I didn't supply.
Thank you.
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Sandy B.
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Received on Tue Feb 16 2016 - 21:06:17 CET