From: Tom Lane <tgl@sss.pgh.pa.us>
To: Heikki Linnakangas <hlinnaka@iki.fi>
cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Polyphase merge is obsolete
In-reply-to: <420a0ec7-602c-d406-1e75-1ef7ddc58d83@iki.fi>
References: <420a0ec7-602c-d406-1e75-1ef7ddc58d83@iki.fi>
Comments: In-reply-to Heikki Linnakangas <hlinnaka@iki.fi>
	message dated "Wed, 12 Oct 2016 20:16:07 +0300"
Date: Wed, 12 Oct 2016 13:27:25 -0400
Message-ID: <1162.1476293245@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org

Heikki Linnakangas <hlinnaka@iki.fi> writes:
> The beauty of the polyphase merge algorithm is that it allows reusing 
> input tapes as output tapes efficiently ... So the whole idea of trying to 
> efficiently reuse input tapes as output tapes is pointless.

It's been awhile since I looked at that code, but I'm quite certain that
it *never* thought it was dealing with actual tapes.  Rather, the point of
sticking with polyphase merge was that it allowed efficient incremental
re-use of temporary disk files, so that the maximum on-disk footprint was
only about equal to the volume of data to be sorted, rather than being a
multiple of that.  Have we thrown that property away?

			regards, tom lane


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