developers-list Digest, Vol 3, Issue 7

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Today's Topics:

1. BugID=3178: A patch to compute stdev using one scan of BAT is
available (Yinhe Cao)


----------------------------------------------------------------------

Message: 1
Date: Tue, 13 Nov 2012 13:06:22 -0800
From: Yinhe Cao <yinhqiao-***@public.gmane.org>
To: developers-list-d/***@public.gmane.org
Subject: BugID=3178: A patch to compute stdev using one scan of BAT is
available
Message-ID:
<CAGuRnr38gLkQbZKZDgNi+5vv1dsRbgewoPh1NtittsWczdD8xg-JsoAwUIsXosN+***@public.gmane.org>
Content-Type: text/plain; charset="iso-8859-1"

Hi,

The current implementation of ***@1
in monetdb5/modules/kernel/algebra.mx
takes two scans of the BAT.
First scan to compute the sum (average)
Second scan to compute stdev.

By using a well-known mathematical property of the stdev, one scan is enough:
during the scan, the sum of the value and the square of the value can
be computed together, and
the stdev can be computed then. The new implementation would roughly
cut the running time by
half for most of meaningful cases, especially when the bat is big.


Link to the patch:
http://bugs.monetdb.org/attachment.cgi?id=158&action=edit

Link to the bug:
http://bugs.monetdb.org/show_bug.cgi?id=3178

Any comments or suggestions will be greatly appreciated.

Thanks,
Yinhe
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