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Re: Parameterized-path cost comparisons need some work

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Parameterized-path cost comparisons need some work
Date: 2012-02-29 19:08:43
Message-ID: 4952.1330542523@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, Feb 29, 2012 at 1:40 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Indeed, and the code already knows that. However, in this example, path
>> A is capable of dominating the other three (being strictly less
>> parameterized than them), and B and C are each capable of dominating D.
>> The problem is just that I'd neglected to consider that rowcount now
>> also becomes a figure of merit.

> In theory, yes, but in practice, won't it nearly always be the case
> that a less parameterized path generates more rows, and a more
> parameterized path generates less rows, so that neither dominates the
> other?

I think you're just assuming that without any solid evidence.  My point
is precisely that if the more-parameterized path *fails* to generate
fewer rows, we want add_path to notice that and throw it out (or at
least be able to throw it out, if there's not another reason to keep it).

			regards, tom lane

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