On Tue, 2008-09-23 at 09:13 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > Thinks: Why not just sort all of the time and skip the debate entirely?
>
> The sort is demonstrably a loser for smaller indexes. Admittedly,
> if the index is small then the sort can't cost all that much, but if
> the (correct) threshold is some large fraction of shared_buffers then
> it could still take awhile on installations with lots-o-buffers.
The other realisation is that for large indexes, giving them more
maintenance_work_mem probably will make them build faster 'cos we'll be
sorting. So "give big indexes more memory" is still true *enough* to be
broadly consistent, explainable and understandable. I do explain things
in more detail on some courses, but pithy rules help busy people.
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support