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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
Message-ID: AANLkTi=D66fO75S2meJNtXz9z81EWW-CJmqh-6Q-8Sob@mail.gmail.com (view raw or flat)
Thread:
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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company

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