| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
| Cc: | Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com>, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz>, Jeff Davis <pgsql(at)j-davis(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, Amit Langote <amitlangote09(at)gmail(dot)com>, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, Fabrízio Mello <fabriziomello(at)gmail(dot)com>, Thom Brown <thom(at)linux(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Parallel Seq Scan |
| Date: | 2015-09-26 14:16:58 |
| Message-ID: | CA+Tgmob9CxCP5-_pFfWiZvZQTA3=78S17UwCMNJFu7QaBcQ6PA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Sat, Sep 26, 2015 at 8:38 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> QueryDesc's totaltime is for instrumentation information for plugin's
>> like pg_stat_statements and we need only the total buffer usage
>> of each worker to make it work as the other information is already
>> collected in master backend, so I think that should work as I have
>> written.
>
> I don't think that's right at all. First, an extension can choose to
> look at any part of the Instrumentation, not just the buffer usage.
> Secondly, the buffer usage inside QueryDesc's totaltime isn't the same
> as the global pgBufferUsage.
Oh... but I'm wrong. As long as our local pgBufferUsage gets update
correctly to incorporate the data from the other workers, the
InstrStopNode(queryDesc->totaltime) will suck in those statistics.
And the only other things getting updated are nTuples (which shouldn't
count anything that the workers did), firsttuple (similarly), and
counter (where the behavior is a bit more arguable, but just counting
the master's wall-clock time is at least defensible). So now I think
you're right: this should be OK.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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