Re: Removing unneeded self joins

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Removing unneeded self joins
Date: 2018-05-16 16:08:46
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Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru> writes:
> There is a join optimization we don't do -- removing inner join of a
> table with itself on a unique column. Such joins are generated by
> various ORMs, so from time to time our customers ask us to look into
> this. Most recently, it was discussed on the list in relation to an
> article comparing the optimizations that some DBMS make [1].

This is the sort of thing that I always wonder why the customers don't
ask the ORM to stop generating such damfool queries. Its *expensive*
for us to clean up after their stupidity; almost certainly, it would
take far fewer cycles, net, for them to be a bit smarter in the first

regards, tom lane

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