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 "Mon, 11 Jun 2018 13:55:33 +1200"
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Date: Mon, 11 Jun 2018 10:26:32 -0400
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David Rowley <david.rowley@2ndquadrant.com> writes:
> On 11 June 2018 at 10:49, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> * The business with ExecFindInitialMatchingSubPlans remapping the
>> subplan indexes seems very dubious to me.  Surely, that is adding
>> way more complexity and fragility than it's worth.

> I think having the ability to prune during executor initialisation is
> enormously important. I think without it, this patch is less than half
> as useful.

Sure.

> However, if you didn't mean removing the executor
> initialise pruning, and just not remapping the arrays, then I'm not
> sure how we'd do that.  I'd previously tried having NULL subnodes in
> the Append and I thought it required too much hacking work in
> explain.c, so I decided to just collapse the array instead and do what
> was required to make it work after having removed the unneeded
> subplans. Did you have another idea on how to do this?

No, that was pretty much exactly what I was envisioning.  I have
not looked at the consequences for explain.c but it seemed like
it couldn't be all that bad; and to my mind the remapping business
in the partition code is pretty bad.  "It's all in one file" is not
a suitable justification for unintelligible, overcomplex code.

			regards, tom lane