Date: Mon, 26 Mar 2018 12:36:13 +0200
From: Peter <pmc@citylink.dinoex.sub.org>
To: Laurenz Albe <laurenz.albe@cybertec.at>
Cc: pgsql-performance@lists.postgresql.org, Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: Should from_collapse be switched off? (queries 10 times faster)
Message-ID: <20180326103612.GA16180@gate.oper.dinoex.org>
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Laurenz,

thank You very much for Your comments!

On Sun, Mar 25, 2018 at 07:12:08AM +0200, Laurenz Albe wrote:

! Your reported execution times don't match the time reported in the
! EXPLAIN output...

Should these match? 
It seems the EXPLAIN (ANALYZE, BUFFERS) does additional things, not 
just execute the query. 

! The cause of the long execution time is clear:
! 
! The row count of the join between "places" (WHERE platz = 'WAEHR'),
! "wpnames" and "places AS places_1" is underestimated by a factor of 10
! (1 row instead of 10).
! 
! The nested loop join that is chosen as a consequence is now executed
! 10 times instead of the estimated 1 time, which is where almost all the
! execution time is spent.

I've seen this, but do not fully understand it yet.
 
! Setting "enable_nestloop = off" is as coarse as forcing "from_collapse = 1"
! and will negatively impact other queries - if it helps at all.

Since this query is already put into a function, I found I can easily
set from_collapse=1 only for this function, by means of "ALTER
FUNCTION ... SET ...", so it does only influence this query. 
It seems this is the most straight-forward solution here.
 
rgds,
P.