From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: James Coleman <jtc331@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@lists.postgresql.org>
Cc: Simon Riggs <simon@2ndquadrant.com>
Subject: Re: PROC_IN_ANALYZE stillborn 13 years ago
Date: Thu, 06 Aug 2020 17:35:33 -0400
Message-ID: <2647180.1596749733@sss.pgh.pa.us> (raw)
In-Reply-To: <CA+TgmoZ9hycF=fu0V+812fOvTsje5bCV6vMdDWphSANOiv-vqw@mail.gmail.com>
References: <20200805021435.GA9523@alvherre.pgsql>
<20200805235549.GA8118@alvherre.pgsql>
<20200806010711.mzvu36zy25xer33t@alap3.anarazel.de>
<CA+TgmoZn5oX4j3KXMkQx=RN+QBgc_y3+1LjBcy_afOmJb-f0-A@mail.gmail.com>
<2637311.1596739058@sss.pgh.pa.us>
<CA+Tgmob0veNwAikDiuWTHcTpQHdqAHFqqgf9ycg9dZyiWR30eg@mail.gmail.com>
<2638833.1596741087@sss.pgh.pa.us>
<CA+TgmoZ9hycF=fu0V+812fOvTsje5bCV6vMdDWphSANOiv-vqw@mail.gmail.com>
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
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, robertmhaas@gmail.com, andres@anarazel.de, alvherre@2ndquadrant.com, jtc331@gmail.com, pgsql-hackers@lists.postgresql.org, simon@2ndquadrant.com
Subject: Re: PROC_IN_ANALYZE stillborn 13 years ago
In-Reply-To: <2647180.1596749733@sss.pgh.pa.us>
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