Re: Not sure why parallel hint didn't work correctly
Date: Wed, 17 Feb 2016 08:33:55 -0700
Message-ID: <CAJzM94AmZnrgkuTwGY+3oQhceiq7s-PzHdkeKukJgdujnVTMRA_at_mail.gmail.com>
Got a chance to rerun with parallel(*alias* 48). It performed as well as the test not using a hint. Curiously, everything showed it requested and actual DOP of 32 and not 48. 32 is what we have on the large table. Duration is just over 12 minutes. Without the *alias*, the second server sessions were inactive the entire run. Of the four tables in the query, the two smallest have a degree of 1. The other two have a degree of 32. Parallel query was used only on the one table without the hint and using (alias 48). The run with just parallel(48) does not show parallel query being used for any of the tables. v$sessstat shows all the parallel sessions doing roughly the same amount of activity.
Unfortunately, I am not at liberty to share the SQL statement in this instance (company policy). I was just curious as to what would cause this and possible resolution other than what I did. The runtime without the hint is acceptable to the enduser. He was under the impression he always had to use a hint and the higher the parallel value, the faster his query would run.
Sandy
On Tue, Feb 16, 2016 at 7:20 PM, Chitale, Hemant K <Hemant-K.Chitale_at_sc.com> wrote:
> >>When I put the hint back in, v$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$session_wait showed only one
> session actively doing any work for this query.
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> What was the duration of the query execution ? Did you see INACTIVE and
> ACTIVE for the entire duration (i.e. repeated queries on V$SESSION) ? Did
> V$SESSTAT statistics indicate the level of activity for each of the
> sessions ?
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> > along with some joins to much smaller tables to get to the right set of
> rows
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> Did it show parallel query being used for the joined tables ? In all
> three cases (User query, unhinted query, query with parallel (48)) ?
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> Hemant K Chitale
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> *From:* oracle-l-bounce_at_freelists.org [mailto:
> oracle-l-bounce_at_freelists.org] *On Behalf Of *Sandra Becker
> *Sent:* Wednesday, February 17, 2016 4:06 AM
> *To:* oracle-l
> *Subject:* Not sure why parallel hint didn't work correctly
>
>
>
> Oracle EE 11.2.0.4
>
> This is an Exadata with RAC database.
>
> Database servers - 48 cores
>
>
> I am just starting to learn about Exadata so I may be asking "silly"
> questions. User had a query with hint parallel(48) and looking only at a
> single partition with 1.6 billion rows in that partition, along with some
> joins to much smaller tables to get to the right set of rows. It was not
> returning results in a timely manner. The user indicated he let it run for
> over an hour without any results being returned so he killed it.
>
> I did some testing and looked v$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.
>
>
> --
>
> Sandy B.
>
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-- Sandy B. -- http://www.freelists.org/webpage/oracle-lReceived on Wed Feb 17 2016 - 16:33:55 CET