From: Tom Lane <tgl@sss.pgh.pa.us>
To: Simon Riggs <simon@2ndquadrant.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: pgsql-performance@postgresql.org
Subject: Re: hashjoin chosen over 1000x faster plan
Date: Wed, 10 Oct 2007 14:07:30 -0400
Message-ID: <20980.1192039650@sss.pgh.pa.us> (raw)
In-Reply-To: <1192035955.4233.306.camel@ebony.site>
References: <470B99BE.EE98.0025.0@wicourts.gov>
<1191997904.4233.125.camel@ebony.site>
<470C9813.EE98.0025.0@wicourts.gov>
<1192035955.4233.306.camel@ebony.site>
Simon Riggs <simon@2ndquadrant.com> writes:
> Basically the planner doesn't ever optimise for the possibility of the
> never-executed case because even a single row returned would destroy
> that assumption.
It's worse than that: the outer subplan *does* return some rows.
I suppose that all of them had NULLs in the join keys, which means
that (since 8.1 or so) nodeMergejoin discards them as unmatchable.
Had even one been non-NULL the expensive subplan would have been run.
This seems like too much of a corner case to justify adding a lot of
machinery for.
regards, tom lane
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, simon@2ndquadrant.com, Kevin.Grittner@wicourts.gov
Subject: Re: hashjoin chosen over 1000x faster plan
In-Reply-To: <20980.1192039650@sss.pgh.pa.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox