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Re: Parameterized-path cost comparisons need some work

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Parameterized-path cost comparisons need some work
Date: 2012-02-29 19:02:42
Message-ID: CA+Tgmobp-nAtPjxGCwhVq1=Zn1XXKpfj-G2jcyY6KmHOsOS-qg@mail.gmail.com (view raw or flat)
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On Wed, Feb 29, 2012 at 1:40 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I am having trouble
>> constructing an example, but I feel like there might be cases where
>> it's possible to have path A, not parameterized, path B, parameterized
>> by R, and path C, parameterized by S, and maybe also path D,
>> parameterized by both R and S.  In that case, I feel like paths B and
>> C are incomparable.
>
> 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?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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