From: Tom Lane <tgl@sss.pgh.pa.us>
To: Laurenz Albe <laurenz.albe@cybertec.at>
cc: Peter <pmc@citylink.dinoex.sub.org>,
        pgsql-performance@lists.postgresql.org
Subject: Re: Should from_collapse be switched off? (queries 10 times faster)
In-reply-to: <1521805295.2471.40.camel@cybertec.at>
References: <20180323100308.GA1188@gate.oper.dinoex.org>
 <1521805295.2471.40.camel@cybertec.at>
Comments: In-reply-to Laurenz Albe <laurenz.albe@cybertec.at>
	message dated "Fri, 23 Mar 2018 12:41:35 +0100"
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Date: Fri, 23 Mar 2018 10:14:19 -0400
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Laurenz Albe <laurenz.albe@cybertec.at> writes:
> Peter wrote:
>> I could not find any documentation or evaluation that would say
>> that from_collapse can have detrimental effects. Even less, which
>> type of queries may suffer from that.

> https://www.postgresql.org/docs/current/static/explicit-joins.html
> states towards the end of the page that the search tree grows
> exponentially with the number of relations, and from_collapse_limit
> can be set to control that.

It's conceivable that the OP's problem is actually planning time
(if the query joins sufficiently many tables) and that restricting
the cost of the join plan search is really what he needs to do.
Lacking any further information about the problem, we can't say.

We can, however, point to

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

concerning how to ask this type of question effectively.

			regards, tom lane