Re: why partition pruning doesn't work?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
Cc: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: why partition pruning doesn't work?
Date: 2018-06-11 14:26:32
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David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
> On 11 June 2018 at 10:49, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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.


> 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

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