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Subject: Re: Removing unneeded self joins
From: "Jonathan S. Katz" <jkatz@postgresql.org>
In-Reply-To: <20180516185840.a7crgjho7txxqmi4@alap3.anarazel.de>
Date: Wed, 16 May 2018 15:36:51 -0500
Cc: Robert Haas <robertmhaas@gmail.com>,
 Tom Lane <tgl@sss.pgh.pa.us>,
 Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>
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References: <64486b0b-0404-e39e-322d-0801154901f3@postgrespro.ru>
 <3295.1526486926@sss.pgh.pa.us>
 <CA+TgmoYuH_Tkdv7_fxOinJLf7gw6NyYq1Lm=gevq8aoj507g4A@mail.gmail.com>
 <20180516185840.a7crgjho7txxqmi4@alap3.anarazel.de>
To: Andres Freund <andres@anarazel.de>
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> On May 16, 2018, at 1:58 PM, Andres Freund <andres@anarazel.de> wrote:
>=20
> Hi,
>=20
> On 2018-05-16 12:26:48 -0400, Robert Haas wrote:
>> Also, I'm not sure that I believe that it's always easy to avoid
>> generating such queries.
>=20
> Yea. There's obviously plenty cases where ORMs just want to make the
> database hurt. But especially when building a join between a number of
> tables based on various fields, it's not going to be easy for the ORM =
to
> figure out which ones can be safely omitted. It'd need similar
> optimization as we'd have to do, without having the infrastructure =
core
> PG has.  And then there's, as you say, views etc=E2=80=A6

Are there specific examples of what the ORM code is that generated
the SQL? I=E2=80=99m more curious to see what people are writing that
generates such code. As earlier mentioned we could always report back
to the specific ORM maintainer(s) such examples and see if they could
tweak.

Jonathan=