Re: Polyphase merge is obsolete

On 10/12/2016 08:27 PM, Tom Lane wrote:
> 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?

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