From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
Cc: | David Rowley <dgrowleyml(at)gmail(dot)com>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Jim Nasby <jim(at)nasby(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Patch to support SEMI and ANTI join removal |
Date: | 2014-09-29 14:12:25 |
Message-ID: | 11640.1411999945@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On 2014-09-28 10:41:56 -0400, Tom Lane wrote:
>> If this optimization only works in that scenario, it's dead in the water,
>> because that assumption is unsupportable. The planner does not in general
>> use the same query snapshot as the executor, so even in an immediate-
>> execution workflow there could have been data changes (caused by other
>> transactions) between planning and execution.
> I don't think the effects of other queries are the problem here. The
> effect of other backend's deferred FK checks shouldn't matter for other
> backends for normal query purposes. It's the planning backend that might
> have deferred checks and thus temporarily violated foreign keys.
I see. So why aren't we simply ignoring deferrable FKs when making the
optimization? That pushes it back from depending on execution-time state
(unsafe) to depending on table DDL (safe).
regards, tom lane
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