| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Kevin Grittner <kgrittn(at)gmail(dot)com> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Kevin Grittner <kgrittn(at)postgresql(dot)org>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, "pgsql-committers(at)postgresql(dot)org" <pgsql-committers(at)postgresql(dot)org> |
| Subject: | Re: Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold < |
| Date: | 2016-05-24 18:12:06 |
| Message-ID: | 6588.1464113526@sss.pgh.pa.us |
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| Lists: | pgsql-committers pgsql-hackers |
Kevin Grittner <kgrittn(at)gmail(dot)com> writes:
> On Tue, May 24, 2016 at 12:00 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> Analyze IIRC acquires a new snapshot when getting sample rows,
> I could not find anything like that, and a case-insensitive search
> of analyze.c finds no occurrences of "snap". Can you remember
> where you think you saw something that would cause the ANALYZE
> command in my test to use a snapshot other than the one from the
> REPEATABLE READ transaction in which it was run?
The control logic concerned with that is in vacuum.c, not analyze.c.
AFAICS a manual ANALYZE that's within a transaction block would use
the prevailing snapshot (see in_outer_xact tests). There are a lot
of cases that wouldn't, though.
regards, tom lane
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