Re: [HACKERS] Re: Improve OR conditions on joined columns (common star schema problem)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Michael Paquier <michael(at)paquier(dot)xyz>
Cc: Noah Misch <noah(at)leadboat(dot)com>, Jim Nasby <jim(dot)nasby(at)openscg(dot)com>, pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: [HACKERS] Re: Improve OR conditions on joined columns (common star schema problem)
Date: 2018-10-02 01:32:10
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Michael Paquier <michael(at)paquier(dot)xyz> writes:
> On Mon, Sep 03, 2018 at 06:59:10PM -0700, Noah Misch wrote:
>> If you're going to keep this highly-simplified estimate, please expand the
>> comment to say why it doesn't matter or what makes it hard to do better. The
>> non-planunionor.c path for the same query computes its own estimate of the
>> same underlying quantity. Though it may be too difficult in today's system,
>> one could copy the competing path's row count estimate here. Perhaps it
>> doesn't matter because higher-level processing already assumes equal row count
>> among competitors?

> As there has been no replies to Noah's review for one month, I am
> marking this patch as returned with feedback for now.

FWIW, my problem with this patch is that I remain unconvinced of the basic
correctness of the transform (specifically the unique-ification approach).
Noah's points would be important to address if we were moving the patch
towards commit, but I don't see much reason to put effort into it until
we can think of a way to prove whether that works.

regards, tom lane

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