| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Martijn van Oosterhout <kleptog(at)gmail(dot)com> |
| Cc: | Richard Guo <guofenglinux(at)gmail(dot)com>, PostgreSQL mailing lists <pgsql-bugs(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: BUG #17564: Planner bug in combination of generate_series(), unnest() and ORDER BY |
| Date: | 2022-08-03 21:53:30 |
| Message-ID: | 3243867.1659563610@sss.pgh.pa.us |
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| Lists: | pgsql-bugs |
Martijn van Oosterhout <kleptog(at)gmail(dot)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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