[prev in list] [next in list] [prev in thread] [next in thread]
List: pgsql-hackers
Subject: Re: [HACKERS] Parameterized-path cost comparisons need some work
From: Tom Lane <tgl () sss ! pgh ! pa ! us>
Date: 2012-02-29 23:01:44
Message-ID: 25460.1330556504 () sss ! pgh ! pa ! us
[Download RAW message or body]
Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Feb 29, 2012 at 2:08 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 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).
> Well, my "evidence" is that a parameterized path should pretty much
> always include a paramaterized path somewhere in there - otherwise,
> what is parameterization doing for us?
Well, yes, we know that much.
> And that's going to reduce the
> row count. I may be missing something, but I'm confused as to why
> this isn't nearly tautological.
We don't know that --- I will agree it's likely, but that doesn't make
it so certain that we can assume it without checking. A join condition
won't necessarily eliminate any rows.
(... thinks about that for awhile ...) One thing we could possibly do
is have indxpath.c arbitrarily reject parameterizations that don't
produce a smaller estimated number of rows than an unparameterized scan.
Admittedly, this still doesn't *prove* the assumption for join
relations, but maybe it brings the odds to where it's okay for add_path
to make such an assumption.
(... thinks some more ...) No, that doesn't get us there, because that
doesn't establish that a more-parameterized path produces fewer rows
than some path that requires less parameterization, yet not none at
all. You really want add_path carrying out those comparisons. In your
previous example, it's entirely possible that path D is dominated by B
or C because of poor choices of join quals.
regards, tom lane
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
[prev in list] [next in list] [prev in thread] [next in thread]
Configure |
About |
News |
Add a list |
Sponsored by KoreLogic