From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> |
Cc: | Josh Berkus <josh(at)agliodbs(dot)com>, Gregory Stark <stark(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Hans-Jürgen Schönig <postgres(at)cybertec(dot)at> |
Subject: | Re: Mini improvement: statement_cost_limit |
Date: | 2008-08-15 16:12:17 |
Message-ID: | 200808151612.m7FGCHb25428@momjian.us |
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Lists: | pgsql-hackers |
Ron Mayer wrote:
> Bruce Momjian wrote:
> > Josh Berkus wrote:
> >> ...simple web applications, where
> >> queries are never supposed to take more than 50ms. If a query turns up
> >> with an estimated cost of 10000000000, then you know something's wrong;
> >> ...
> >
> > How about a simpler approach that throws an error or warning for
> > cartesian products? That seems fool-proof.
>
> Seems less fool-proof to me.
>
> Sometimes cartesian products produce plans that run 200 times
> faster than plans that don't use the cartesian product.
>
> The first link below shows a cartesian join that took 1.1
> seconds (within the range of OK for some web apps), while
> plans for the same query that don't use one took 200 seconds.
>
> http://archives.postgresql.org/pgsql-performance/2008-03/msg00391.php
> http://archives.postgresql.org/pgsql-performance/2007-12/msg00090.php
> http://archives.postgresql.org/pgsql-performance/2008-03/msg00361.php
My point is that people should _know_ they are using a cartesian
product, and a warning would do that for users who have no need for a
cartesian product and want to be warned about a possible error.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
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