| From: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Removing unneeded self joins |
| Date: | 2018-05-16 22:24:53 |
| Message-ID: | CAKJS1f9UkZgezi0Uj0P+0HhJsZ2KZEPaGL8ch5w8SA3p8GFGQA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On 17 May 2018 at 10:13, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Yeah. It'd have to be a very heuristic thing that doesn't account
> for much beyond the number of relations in the query, and maybe their
> sizes --- although I don't think we even know the latter at the
> point where join removal would be desirable. (And note that one of
> the desirable benefits of join removal is not having to find out the
> sizes of removed rels ... so just swapping that around doesn't appeal.)
There's probably some argument for delaying obtaining the relation
size until after join removal and probably partition pruning too, but
it's currently done well before that in build_simple_rel, where the
RelOptInfo is built.
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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