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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Martijn van Oosterhout <kleptog@gmail.com>
Cc: Richard Guo <guofenglinux@gmail.com>
Cc: PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Subject: Re: BUG #17564: Planner bug in combination of generate_series(), unnest() and ORDER BY
Date: Wed, 03 Aug 2022 17:53:30 -0400
Message-ID: <3243867.1659563610@sss.pgh.pa.us> (raw)
In-Reply-To: <CADWG95uxFuobe8AoXdYLCTzDBWo3JUkHS7m6J=X4m6qdn4CkTQ@mail.gmail.com>
References: <17564-c7472c2f90ef2da3@postgresql.org>
	<2184335.1659392231@sss.pgh.pa.us>
	<CADWG95s03YZsRy3+q0bQFXW_v_f-9vMvcFOo6KUL08DvqbXhqg@mail.gmail.com>
	<CAMbWs49AgGUophtyThqHhSUyDTu3h6oATg-7GgowCtvHWCh9qQ@mail.gmail.com>
	<3002973.1659538573@sss.pgh.pa.us>
	<CADWG95uxFuobe8AoXdYLCTzDBWo3JUkHS7m6J=X4m6qdn4CkTQ@mail.gmail.com>

Martijn van Oosterhout <kleptog@gmail.com> writes:
> The part I haven't seen explained is why the generate_series() is
> important. My guess is that if you replace it with an expression it is no
> longer an SRF and it produces some completely different plan that prevents
> the problematic path being triggered.

I think the generate_series() forces a ProjectSet to be put atop the
join, where we'd probably not have done that without it, having made
the bogus assumption that we could evaluate the sort-key SRF at scan
level.  Why that changes the costs enough to mask or not mask the bug
is still obscure.

			regards, tom lane





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  Subject: Re: BUG #17564: Planner bug in combination of generate_series(), unnest() and ORDER BY
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