From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Andres Freund <andres@anarazel.de>,
        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 Robert Haas <robertmhaas@gmail.com>
	message dated "Thu, 06 Aug 2020 16:22:23 -0400"
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Date: Thu, 06 Aug 2020 17:35:33 -0400
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Robert Haas <robertmhaas@gmail.com> writes:
> ... how
> important is stability to ANALYZE? If you *either* retake your MVCC
> snapshots periodically as you re-scan the table *or* use a non-MVCC
> snapshot for the scan, you can get those same kinds of artifacts: you
> might see two copies of a just-updated row, or none. Maybe this would
> actually *break* something - e.g. could there be code that would get
> confused if we sample multiple rows for the same value in a column
> that has a UNIQUE index? But I think mostly the consequences would be
> that you might get somewhat different results from the statistics.

Yeah, that's an excellent point.  I can imagine somebody complaining
"this query clearly matches a unique index, why is the planner estimating
multiple rows out?".  But most of the time it wouldn't matter much.
(And I think you can get cases like that anyway today.)

> It's not clear to me that it would even be correct to categorize those
> somewhat-different results as "less accurate."

Estimating two rows where the correct answer is one row is clearly
"less accurate".  But I suspect you'd have to be quite unlucky to
get such a result in practice from Simon's proposal, as long as we
weren't super-aggressive about changing ANALYZE's snapshot a lot.

			regards, tom lane