From: Tom Lane <tgl@sss.pgh.pa.us>
To: Marco Di Cesare <Marco.DiCesare@pointclickcare.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Query with large number of joins
Date: Mon, 20 Oct 2014 19:59:57 -0400
Message-ID: <24746.1413849597@sss.pgh.pa.us> (raw)
In-Reply-To: <e187bede699941a4b3fe6781a69ecd14@CO2PR0701MB759.namprd07.prod.outlook.com>
References: <e187bede699941a4b3fe6781a69ecd14@CO2PR0701MB759.namprd07.prod.outlook.com>
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Marco Di Cesare <Marco.DiCesare@pointclickcare.com> writes:
> We are using a BI tool that generates a query with an unusually large number of joins. My understanding is that with this many joins Postgres query planner can't possibly use an exhaustive search so it drops into a heuristics algorithm. Unfortunately, the query runs quite slow (~35 seconds) and seems to ignore using primary keys and indexes where available.
> Query plan here (sorry had to anonymize):
> http://explain.depesz.com/s/Uml
It's difficult to make any detailed comments when you've shown us only an
allegedly-bad query plan, and not either the query itself or the table
definitions.
However, it appears to me that the query plan is aggregating over a rather
large number of join rows, and there are very few constraints that would
allow eliminating rows. So I'm not at all sure there is a significantly
better plan available. Are you claiming this query was instantaneous
on SQL Server?
The only thing that jumps out at me as possibly improvable is that with
a further increase in work_mem, you could probably get it to change the
last aggregation step from Sort+GroupAggregate into HashAggregate,
which'd likely run faster ... assuming you can spare some more memory.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, Marco.DiCesare@pointclickcare.com
Subject: Re: Query with large number of joins
In-Reply-To: <24746.1413849597@sss.pgh.pa.us>
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