| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | David Rowley <dgrowleyml(at)gmail(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Jim Nasby <jim(at)nasby(dot)net>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Patch to support SEMI and ANTI join removal |
| Date: | 2014-09-12 13:38:43 |
| Message-ID: | 29805.1410529123@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> On Fri, Sep 12, 2014 at 3:35 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> I haven't read the patch, but I think the question is why this needs
>> to be different than what we do for left join removal.
> I discovered over here ->
> http://www.postgresql.org/message-id/CAApHDvo5wCRk7uHBuMHJaDpbW-b_oGKQOuisCZzHC25=H3__fA@mail.gmail.com
> during the early days of the semi and anti join removal code that the
> planner was trying to generate paths to the dead rel. I managed to track
> the problem down to eclass members still existing for the dead rel. I guess
> we must not have eclass members for outer rels? or we'd likely have seen
> some troubles with left join removals already.
Mere existence of an eclass entry ought not cause paths to get built.
It'd be worth looking a bit harder into what's happening there.
regards, tom lane
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