On 10/12/2016 08:27 PM, Tom Lane wrote:
Heikki Linnakangas <hlinn...@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?

No, there's no difference to that behavior. logtape.c takes care of incremental re-use of disk space, regardless of the merging pattern.

- Heikki



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