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Re: wip: functions median and percentile

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Cc: Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>, Greg Stark <gsstark(at)mit(dot)edu>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, David Fetter <david(at)fetter(dot)org>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: wip: functions median and percentile
Date: 2010-10-04 17:22:39
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Lists: pgsql-hackerspgsql-rrreviewers
On Mon, Oct 4, 2010 at 2:58 AM, Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> wrote:
> That requires a new sort for each row. I generated this with a minor
> tweak to Pavel's patch to just restart the tuplesort each time (the
> "quick-fix" solution). The problem is that performance really sucks,
> because it is an O(n^2 log(n)) algorithm.

Maybe that's OK.  If you're doing repeated median operations on large
data sets, perhaps you should expect that to be slow.  I bet that
people who want to use this as a window function will want one median
per group, not n medians per group; and it doesn't seem like a good
idea to say - we're not going to let you use this as a window function
AT ALL because you might decide to do something that will be really
slow.  You can always hit ^C if you get tired of waiting.  This seems
like it's very far from being the most important thing for us to
optimize, though of course it's great if we can.

Robert Haas
The Enterprise Postgres Company

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