From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Wheeler <dwheeler@dgitsystems.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Queue table that quickly grows causes query planner to choose poor plan
Date: Wed, 27 Jun 2018 14:27:33 -0400
Message-ID: <24443.1530124053@sss.pgh.pa.us> (raw)
In-Reply-To: <64CC9527-C642-4AED-844B-70E7BC996A1B@dgitsystems.com>
References: <64CC9527-C642-4AED-844B-70E7BC996A1B@dgitsystems.com>
David Wheeler <dwheeler@dgitsystems.com> writes:
> I'm having performance trouble with a particular set of queries. It goes a bit like this
> 1) queue table is initially empty, and very narrow (1 bigint column)
> 2) we insert ~30 million rows into queue table
> 3) we do a join with queue table to delete from another table (delete from a using queue where a.id<http://a.id; = queue.id<http://queue.id;), but postgres stats say that queue table is empty, so it uses a nested loop over all 30 million rows, taking forever
Although there's no way to have any useful pg_statistic stats if you won't
do an ANALYZE, the planner nonetheless can see the table's current
physical size, and what it normally does is to multiply the last-reported
tuple density (reltuples/relpages) by the current size. So if you're
getting an "empty table" estimate anyway, I have to suppose that the
table's state involves reltuples = 0 and relpages > 0. That's not a
good place to be in; it constrains the planner to believe that the table
is in fact devoid of tuples, because that's what the last ANALYZE saw.
Now, the initial state for a freshly-created or freshly-truncated table
is *not* that. It is reltuples = 0 and relpages = 0, representing an
undefined tuple density. Given that, the planner will make some guess
about average tuple size --- which is likely to be a very good guess,
for a table with only fixed-width columns --- and then compute a rowcount
estimate using that plus the observed physical size.
So I think your problem comes from oscillating between really-empty
and not-at-all-empty, and not using an idiomatic way of going back
to the empty state. Have you tried using TRUNCATE instead of DELETE?
> This queue table is empty 99% of the time, and the query in 3 runs immediately after step 2. Is there any likelyhood that tweaking the autoanalyze params would help in this case? I don't want to explicitly analyze the table between steps 2 and three either as there are other patterns of use where for example 0 rows are inserted in step 2 and this is expected to run very very quickly. Do I have any other options?
I am not following your aversion to sticking an ANALYZE in there,
either. It's not like inserting 30 million rows would be free.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, dwheeler@dgitsystems.com
Subject: Re: Queue table that quickly grows causes query planner to choose poor plan
In-Reply-To: <24443.1530124053@sss.pgh.pa.us>
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