On 03/03/15 03:08, Arthur Silva wrote: > Does it always perform mergesort instead of quicksort when enabled?
Yes; there seemed no advantage to any additional complexity. The merge consistently performs fewer comparisons than the quicksort, on random input - and many fewer if there are any sorted (or reverse-sorted) sections. However, it also consistently takes slightly longer (a few percent). I was unable to chase this down but assume it to be a cacheing effect. So I don't currently think it should replace the current sort for all use. If the planner can identify cases where there is some pre-sort in the input (CLUSTER was mentioned?), or maybe a correlated index, it could be worthwhile. Also useful might be very-expensive comparison cases, and distinct-output cases (uniqification is supported at each submerge stage, so data disappears early). There's potential for running submerges in parallel using multiple cores, but I've not worked on that. -- Cheers, Jeremy -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
