Date: Thu, 06 Jan 2022 18:39:08 +0900 (JST)
Message-Id: <20220106.183908.758244498024205160.horikyota.ntt@gmail.com>
To: vijaykumarjain.github@gmail.com
Cc: AviW@gilat.com, pgsql-performance@postgresql.org
Subject: Re: Same query 10000x More Time
From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
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At Thu, 6 Jan 2022 13:50:55 +0530, Vijaykumar Jain <vijaykumarjain.gith=
ub@gmail.com> wrote in =

> On Thu, 6 Jan 2022 at 13:13, Avi Weinberg <AviW@gilat.com> wrote:
> =

> > Hi
> >
> >
> >
> > I have postgres_fdw table called tbl_link.  The source table is 2.5=
 GB in
> > size with 122 lines (some lines has 70MB bytea column, but not the =
ones I
> > select in the example)
> >
> > I noticed that when I put the specific ids in the list "where id in=

> > (140,144,148)" it works fast (few ms), but when I put the same list=
 as
> > select "where id in (select 140 as id union select 144  union selec=
t 148)"
> > it takes 50 seconds.  This select union is just for the example, I
> > obviously have a different select (which by itself takes few ms but=
 cause
> > the whole insert query to take 10000x more time)
> >
> >
> >
> > Why is that?  How can I still use regular select and still get reas=
onable
> > response time?
> >
> >
> >
> > Thanks
> >
> >
> >
> =

> couple of things:
> PostgreSQL: Documentation: 14: F.35. postgres_fdw
> <https://www.postgresql.org/docs/current/postgres-fdw.html>
> <https://www.postgresql.org/docs/current/postgres-fdw.html>when you s=
et
> your foreign server what are your
> use_remote_estimate
> fetch_size
> params for the foreign server.
> =

> you need to know there are certain restrictions on what gets pushed d=
own to
> the remote server
> i generally use postgres/postgres_fdw.sql at master =B7 postgres/post=
gres
> (github.com)
> <https://github.com/postgres/postgres/blob/master/contrib/postgres_fd=
w/sql/postgres_fdw.sql>
> as
> a reference
> if you predicates are not pushed down, it will bring all the rows fro=
m the
> foreign server to your local server (and fetch_size value and network=
 io
> will add to delay)
> and given you used select * , it will be a lot of io, so maybe restri=
ct
> only to columns needed after being filtered would help.
> =

> =

> you can try by running
> explain (verbose,analyze) query  and then also enabling log_statement=
 =3D
> 'all' / log_min_duration_statement =3D 0
> on the foreign server to see the actual plan for the foreign scan.
> =

> That might help in trouble shooting.
> =

> =

> as always, i have little production exposure. If i am wrong, i can be=

> corrected.

In this specific case, the FAST query doesn't contain a join and its
predicate can be pushed down to remote.  On the other hand the SLOW
one contains a join.  The planner considers remote join only when the
both hands of a join are on the same foreign server.  Tthis is not the
case since the inner subquery is not even a foreign scan.  The planner
doesn't consider the possibility that a subquery is executable
anywhere.

As the result, the local inevitably draw all rows from remote table to
join with the result of the subquery on-local, which should be quite
slow.

It could be improved, but I don't think we are going to consider that
case because the SLOW query seems like a kind of bad query, which can
be improved by rewriting to the FAST one.

regards.

-- =

Kyotaro Horiguchi
NTT Open Source Software Center