Re: pgsql-server/src/backend/utils/sort tuplesort.c

Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > Tom Lane wrote:
> >> During btree index build, sort equal-keyed tuples according to their
> >> TID (heap position). This doesn't do anything to the validity of the
> >> finished index, but by pretending to qsort() that there are no really
> >> equal keys in the sort, we can avoid performance problems with qsort
> >> implementations that have trouble with large numbers of equal keys.
> >> Patch from Manfred Koizar.
>
> > I think there is also the advantage that many equal keys will access the
> > heap in a more sequential, rather than random, order, which is the part
> > that really excited me.
>
> But we aren't attempting to maintain that ordering after index build.
> (In fact, it was exactly that point that triggered the argument last
> time round ...)

Agreed, but we don't maintain CLUSTER either. I see no harm in having
it start out ordered, at least.

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