2010/9/23 Hitoshi Harada <umi.tan...@gmail.com>: > 2010/9/23 Pavel Stehule <pavel.steh...@gmail.com>: >> Hello >> >> 2010/9/22 Hitoshi Harada <umi.tan...@gmail.com>: >>> 2010/9/22 Pavel Stehule <pavel.steh...@gmail.com>: >>>> Hello >>>> >>>> I found probably hard problem in cooperation with window functions :( >> >> maybe I was confused. I found a other possible problems. >> >> The problem with median function is probably inside a final function >> implementation. Actually we request possibility of repetitive call of >> final function. But final function call tuplesort_end function and >> tuplesort_performsort. These function changes a state of tuplesort. >> The most basic question is "who has to call tuplesort_end function and >> when? > > Reading the comment in array_userfuncs.c, array_agg_finalfn() doesn't > clean up its internal state at all and tells it's the executor's > responsibility to clear memory. It is allowed since ArrayBuildState is > only in-memory state. In the other hand, TupleSort should be cleared > by calling tuplesort_end() if it has tapeset member (on file based > sort) to close physical files. > > So 2 or 3 ways to go in my mind:
it is little bit worse - we cannot to call tuplesort_performsort repetitive. > > 1. call tuplesort_begin_datum with INT_MAX workMem rather than the > global work_mem, to avoid it spills out sort state to files. It may > sounds dangerous, but actually memory exhausting can happen in > array_agg() as well. > > 2. add TupleSort an argument that tells not to use file at all. This > results in the same as #1 but more generic approach. > > 3. don't use tuplesort in median() but implement its original sort > management. This looks quite redundant and like maintenance problem. > > #2 sounds like the best in generic and consistent way. The only point > is whether the change is worth for implementing median() as it's very > system-wide common fundamentals. > > Other options? #4 block median under window clause #5 use a C array instead tuplesort under window clause. It is very unpractical to use a windows clauses with large datasets, so it should not be a problem. More, this can be very quick, because for C array we can use a qsort function. Now I prefer #5 - it can be fast for using inside windows clause and safe when window clause will not be used. Regards Pavel > > > Regards, > -- > Hitoshi Harada > -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
