Re: Patch to support SEMI and ANTI join removal

Andres Freund <andres@2ndquadrant.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