From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <david.rowley@2ndquadrant.com>
cc: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>,
        Dmitry Dolgov <9erthalion6@gmail.com>,
        Jeff Janes <jeff.janes@gmail.com>,
        Ashutosh Bapat <ashutosh.bapat@enterprisedb.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 David Rowley <david.rowley@2ndquadrant.com>
	message dated "Tue, 12 Jun 2018 02:40:25 +1200"
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Date: Mon, 11 Jun 2018 11:00:14 -0400
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David Rowley <david.rowley@2ndquadrant.com> writes:
> On 12 June 2018 at 02:26, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> ... I'd previously tried having NULL subnodes in
>>> the Append and I thought it required too much hacking work in
>>> explain.c,

>> No, that was pretty much exactly what I was envisioning.

> What you're proposing exchanges logic that fits neatly into one
> function for special logic that will be scattered all over explain.c.
> I really don't think that's the better of the two evils.

As far as I can see, it'd involve about three or four lines' worth of
rewrite in one place-you-already-made-quite-ugly in explain.c, and an
added if-test in planstate_walk_members, and that'd be it.  That seems
like a pretty cheap price for being able to vastly simplify the invariants
for the pruning functions.  In fact, I doubt you'd even need two of them
anymore; just one with a bool flag for can-use-exec-params.

> I'd rather just see my last patch applied which simplifies the re-map
> code by removing the 2nd loop.  Actually, even updating the
> present_parts to remove the unneeded subpartitioned table indexes is
> optional. We' just need to give up on the elog(ERROR, "partition
> missing from subplans"); error and assume that case is fine.

The fact that you added that loop and only later decided it was
unnecessary seems to me to support my point pretty strongly: that
code is overcomplicated.

			regards, tom lane