| From: | David Rowley <dgrowleyml(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Onder Kalaci <onderk(at)microsoft(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Assertion failure with LEFT JOINs among >500 relations |
| Date: | 2020-10-08 23:27:21 |
| Message-ID: | CAApHDvp8vkN-hXdxgvZHKGnZzGneQO5c5kGgsZmuqJMP1rcZmA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Fri, 9 Oct 2020 at 12:16, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> > Perhaps the right fix is to modify clamp_row_est() with:
>
> I thought of that too, but as you say, if the rowcount has overflowed a
> double then we've got way worse problems. It'd make more sense to try
> to keep the count to a saner value in the first place.
I wonder if there was something more logical we could do to maintain
sane estimates too, but someone could surely still cause it to blow up
by writing a long series of clause-less joins. We can't really get
away from the fact that we must estimate those as inner_rows *
outer_rows
I admit it's annoying to add cycles to clamp_row_est() for such insane cases.
David
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