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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 16:57:29
Message-ID: 24934.1250614649@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
I wrote:
> Matthew Wakeling <matthew(at)flymine(dot)org> writes:
>> Very clever. Yes, that is what is happening. I'm surprised that the system 
>> doesn't buffer the inner side to avoid having to rescan each time, but 
>> then I guess you would have problems if the buffer grew larger than 
>> memory.

> Well, it does consider adding a Materialize node for that purpose,
> but in this case it evidently thought a sort was cheaper.

Hmmm ... actually, after looking at the code, I notice that we only
consider adding a Materialize node to buffer an inner input that is a
Sort node.  The idea was suggested by Greg Stark, if memory serves.
I wonder now if it'd be worthwhile to generalize that to consider
adding a Materialize above *any* inner mergejoin input.

			regards, tom lane

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