On Wed, Mar 7, 2012 at 7:55 PM, Robert Haas <robertmh...@gmail.com> wrote: >> But it would mean we have about 1.7x more runs that need to be merged >> (for initially random data). Considering the minimum merge order is >> 6, that increase in runs is likely not to lead to an additional level >> of merging, in which case the extra speed of building the runs would >> definitely win. But if it does cause an additional level of merge, it >> could end up being a loss. > > That's true, but the real hit to the run size should be quite a bit > less than 1.7x, because we'd also be using memory more efficiently, > and therefore more tuples should fit.
I'm not sure I believe the 1.7x. Keep in mind that even after starting a new tape we can continue to read in new tuples that belong on the first tape. So even if you have tuples that are more than N positions out of place (where N is the size of your heap) as long as you don't have very many you can keep writing out the first tape for quite a while. I suspect analyzing truly random inputs is also a bit like saying no compression algorithm can work on average. Partly sorted data is quite common and the tapesort algorithm would be able to do a lot of cases in a single merge that the quicksort and merge would generate a large number of merges for. All that said, quicksort and merge would always do no more i/o in cases where the total number of tuples to be sorted is significantly less than N^2 since that would be guaranteed to be possible to process with a single merge pass. (Where "significantly less" has something to do with how many tuples you have to read in one i/o to be efficient). That probably covers a lot of cases, and Postgres already has the stats to predict when it would happen, more or less. Fwiw when I was doing the top-k sorting I did a bunch of experiements and came up with a rule-of-thumb that our quicksort was about twice as fast as our heapsort. I'm not sure whether that's a big deal or not in this case. Keep in mind that the only win would be reducing the cpu time on a sort where every tuple was being written to disk and read back. For most users that won't run noticeably faster, just reduce cpu time consumption. -- greg -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers