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Re: Weird index or sort behaviour

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Matthew Wakeling <matthew(at)flymine(dot)org>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Weird index or sort behaviour
Date: 2009-08-18 15:05:23
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Matthew Wakeling <matthew(at)flymine(dot)org> writes:
> I'm seeing some interesting behaviour. I'm executing a query where I 
> perform a merge join between two copies of the same table, completely 
> symmetrically, and the two sides of the merge are sourced differently.

This is not as surprising as you think.  A mergejoin is *not*
symmetrical between its two inputs: the inner side is subject to being
partially rewound and rescanned when the outer side is advanced to a new
row with the same merge key.  This means there is a premium on cheap
rescan for the inner side that doesn't exist for the outer ... and a
sort node is cheaper to rescan than a generic indexscan.  It's
impossible to tell from the data you provided whether the planner was
correct to pick a sort over an indexscan for the inner side, but the
fact that it did so is not prima facie evidence of a bug.  You could
force choice of the other plan via enable_sort = off and then compare
estimated and actual runtimes to see if the planner got it right.

			regards, tom lane

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