From: Tom Lane <tgl@sss.pgh.pa.us>
To: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
cc: Dmitry Dolgov <9erthalion6@gmail.com>,
        Pavel Stehule <pavel.stehule@gmail.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: why partition pruning doesn't work?
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Comments: In-reply-to Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
	message dated "Fri, 01 Jun 2018 11:43:55 -0400"
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Date: Fri, 01 Jun 2018 11:53:39 -0400
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Ashutosh Bapat <ashutosh.bapat@enterprisedb.com> writes:
> I think the patch is right if we were to handle only SQLValueFunction,
> but the bigger picture here is that we aren't evaluating stable
> functions before run-time partition pruning happens. I was under the
> impression that the stable functions/expressions get evaluated and
> folded into a constant just before the execution begins since a stable
> function produces the same output for same input during one execution
> invocation. But I am not able to find where we do that and probably we
> don't do that at all.

We don't; there was a patch floating around to make that happen, but
it hasn't been updated lately.

I agree though that it seems strange to special-case SQLValueFunction
rather than any-stable-expression.  As long as the evaluation happens
at executor start (i.e. with the query's run-time snapshot) it should
be reasonable to simplify any stable expression.

It's worth questioning whether this is a bug fix or an improvement.
If the latter, it probably ought to wait for v12.

			regards, tom lane