From: Tom Lane <tgl@sss.pgh.pa.us>
To: Eric Raskin <eraskin@paslists.com>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: Adding nextval() to a select caused hang/very slow execution
Date: Wed, 04 Nov 2020 14:23:28 -0500
Message-ID: <1237916.1604517808@sss.pgh.pa.us> (raw)
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Eric Raskin <eraskin@paslists.com> writes:
> And, of course, your explanation that inserts will not be parallelized must
> be the reason. I will certainly re-vacuum the tables. I wonder why
> auto-vacuum didn't collect better stats. vacuum analyze <table> is all I
> need, right?
Plain ANALYZE is enough to collect stats; but I doubt that'll improve
matters for you. The problem is basically that the planner can't do
anything with a CASE construct, so you end up with default selectivity
estimates for anything involving a CASE, statistics or no statistics.
You need to try to reformulate the query with simpler join conditions.
> As a last resort, what about a PL/PGSQL procedure loop on the query
> result? Since the insert is very few rows relative to the work the select
> has to do, I could just turn the insert.. select.. into a for loop. Then
> the select could be parallel?
Maybe, but you're still skating on a cliff edge. I think it's pure chance
that the parallelized query is working acceptably well; next month with
slightly different conditions, it might not.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, eraskin@paslists.com, pgsql-performance@lists.postgresql.org
Subject: Re: Adding nextval() to a select caused hang/very slow execution
In-Reply-To: <1237916.1604517808@sss.pgh.pa.us>
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