Message-ID: <2e94543c5f92f6902ae51a5e6f4f569558fb548a.camel@cybertec.at>
Subject: Re: Slow planning, fast execution for particular 3-table query
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: David Wheeler <dwheeler@dgitsystems.com>,
	"pgsql-performance@lists.postgresql.org"
	 <pgsql-performance@lists.postgresql.org>
Cc: Cameron Redpath <credpath@dgitsystems.com>
Date: Mon, 04 Nov 2019 05:37:00 +0100
In-Reply-To: <175BE386-F861-4064-8B5D-93E63BD90F13@dgitsystems.com>
References: <175BE386-F861-4064-8B5D-93E63BD90F13@dgitsystems.com>
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On Mon, 2019-11-04 at 03:04 +0000, David Wheeler wrote:
> We’re having trouble working out why the planning time for this particular query is slow
> (~2.5s vs 0.9ms execution time). As you can see below, there are only 3 tables involved
> so it’s hard to imagine what decisions the planner has to make that take so long. After
> 5 runs the prepared-statement code kicks in and it becomes quick, but it’s quite
> infuriating for the first 5 runs given the execution is so much faster. 
> 
> Are you able to give any tips what might be taking so long (and how we might improve it)?
> 
[...]
> Planning time: 2429.682 ms
> 
> Execution time: 0.914 ms

Strange.
Are any of your catalog tables unusually large?

SELECT pg_relation_size(t.oid),
       t.relname
FROM pg_class AS t
   JOIN pg_namespace AS n ON t.relnamespace = n.oid
WHERE t.relkind = 'r'
ORDER BY pg_relation_size(t.oid) DESC
LIMIT 10;

Yours,
Laurenz Albe
-- 
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