Abbreviated keys for Numeric (was: Re: B-Tree support function number 3 (strxfrm() optimization))

From: Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>
To: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Cc: Peter Geoghegan <pg(at)heroku(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Abbreviated keys for Numeric (was: Re: B-Tree support function number 3 (strxfrm() optimization))
Date: 2015-01-26 16:43:01
Message-ID: 87fvayhmtj.fsf@news-spur.riddles.org.uk
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Another spinoff from the abbreviation discussion. Peter Geoghegan
suggested on IRC that numeric would benefit from abbreviation, and
indeed it does (in some cases by a factor of about 6-7x or more, because
numeric comparison is no speed demon).

This patch abbreviates numerics to a weight+initial digits
representation (the details differ slightly between 32bit and 64bit
builds to make the best use of the available bits).

On 32bit, numeric values that are between about 10^-44 and 10^83, and
which differ either in order of magnitude or in the leading 7
significant decimal digits (not base-10000 digits, single decimals) will
get distinct abbreviations. On 64bit the range is 10^-176 to 10^332 and
the first 4 base-10000 digits are kept, thus comparing 13 to 16 decimal
digits. This is expected to be ample for applications using numeric to
store numbers; applications that store things in numeric that aren't
actually numbers might not see the benefit, but I have not found any
detectable slowdown from the patch even on constructed pathological
data.

--
Andrew (irc:RhodiumToad)

Attachment Content-Type Size
numeric_sortsup.patch text/x-patch 15.2 KB

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