From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Robert Haas <robertmhaas@gmail.com>,
        Alvaro Herrera <alvherre@2ndquadrant.com>,
        James Coleman <jtc331@gmail.com>,
        pgsql-hackers <pgsql-hackers@lists.postgresql.org>,
        Simon Riggs <simon@2ndquadrant.com>
Subject: Re: PROC_IN_ANALYZE stillborn 13 years ago
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Thu, 06 Aug 2020 14:45:41 -0700"
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Date: Thu, 06 Aug 2020 18:02:26 -0400
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Andres Freund <andres@anarazel.de> writes:
> In fact using conceptually like a new snapshot for each sample tuple
> actually seems like it'd be somewhat of an improvement over using a
> single snapshot.

Dunno, that feels like a fairly bad idea to me.  It seems like it would
overemphasize the behavior of whatever queries happened to be running
concurrently with the ANALYZE.  I do follow the argument that using a
single snapshot for the whole ANALYZE overemphasizes a single instant
in time, but I don't think that leads to the conclusion that we shouldn't
use a snapshot at all.

Another angle that would be worth considering, aside from the issue
of whether the sample used for pg_statistic becomes more or less
representative, is what impact all this would have on the tuple count
estimates that go to the stats collector and pg_class.reltuples.
Right now, we don't have a great story at all on how the stats collector's
count is affected by combining VACUUM/ANALYZE table-wide counts with
the incremental deltas reported by transactions happening concurrently
with VACUUM/ANALYZE.  Would changing this behavior make that better,
or worse, or about the same?

			regards, tom lane