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Re: wip: functions median and percentile

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: David Fetter <david(at)fetter(dot)org>
Cc: Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: wip: functions median and percentile
Date: 2010-09-23 18:56:53
Message-ID: AANLkTikGt8Z1dEOShTmwCCCOCJvmO94UCnYn9ZLCrbi2@mail.gmail.com (view raw or flat)
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2010/9/23 David Fetter <david(at)fetter(dot)org>:
> On Thu, Sep 23, 2010 at 08:27:38PM +0200, Pavel Stehule wrote:
>> 2010/9/23 Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>:
>> > 2010/9/23 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:
>> >> Hello
>> >>
>> >> 2010/9/22 Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>:
>> >>> 2010/9/22 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)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.
>
> If there's some way to do this using the same code in the windowing
> and non-windowing case, that would be much, much better from an
> architectural point of view.  Single Point of Truth and all that.

We can have a median with support a window clause, but limited to
work_mem, or we can have a unlimited median, but without window
clause. I think, I am able to minimalize a code duplicity - just to
define some envelope over  tuplesort.

The unique code isn't possible there - minimally now we have a two
variants - one for numeric result and second for double. But it is
usual - try to look how much AVG functions are in core.

Regards

Pavel

>
> Cheers,
> David.
> --
> David Fetter <david(at)fetter(dot)org> http://fetter.org/
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