Re: Polyphase merge is obsolete

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Polyphase merge is obsolete
Date: 2016-10-12 17:27:25
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Heikki Linnakangas <hlinnaka(at)iki(dot)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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