From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alessandro Manzoni <manzoni.alessandro4@gmail.com>
cc: pgsql-admin@postgresql.org
Subject: Re: Strange performance problem
In-reply-to: <0f9efbe6-9adc-2732-1257-b4fc0367b0f3@gmail.com>
References: <0f9efbe6-9adc-2732-1257-b4fc0367b0f3@gmail.com>
Comments: In-reply-to Alessandro Manzoni <manzoni.alessandro4@gmail.com>
	message dated "Fri, 25 Jan 2019 18:02:24 +0100"
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Date: Fri, 25 Jan 2019 12:22:02 -0500
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Alessandro Manzoni <manzoni.alessandro4@gmail.com> writes:
> Then I have a complex query that has two complete different 
> performances, in these two cases:

> case a)
> with getfieldone as (select * from tableone where fieldone = 'one')
> select * from getfieldone
> inner join tabletwo on ... [and other joins]

> case b)
> select * from viewone
> inner join tabletwo on ... [and other joins, the same as case a)]

> I expected the same performance, but case a) lasts a few hundreds ms, 
> while case b) lasts more than 12 seconds.

WITH is an optimization fence (at present --- there is discussion
of relaxing that) so it's not exactly surprising that these queries
perform differently.  I am a little surprised though that case a)
is the faster one; usually, preventing the planner from optimizing
the whole query as one problem makes things worse.

Saying more than that would require a lot more detail.

https://wiki.postgresql.org/wiki/Slow_Query_Questions

			regards, tom lane