Re: Performance issue in foreign-key-aware join estimation

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
Subject: Re: Performance issue in foreign-key-aware join estimation
Date: 2019-03-22 13:41:30
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David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
> On Fri, 22 Mar 2019 at 10:10, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I'm unsure how hard we should push to get something like this into v12.
>> I'm concerned that its dependency on list_nth might result in performance
>> regressions in some cases; ...

> However, there's always a danger we find some show-stopper with your
> list reimplementation patch, in which case I wouldn't really like to
> be left with list_skip_forward() in core.

We could always do something like what we've already done with
simple_rel_array and simple_rte_array, ie, replace the eq_classes
List with a manually-managed pointer array. Given the small number
of places that touch that list, it wouldn't be too awful --- but
still, I'd only consider that if the List-reimplementation patch
goes down in flames.

regards, tom lane

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