strpos() && KMP

From: Pavel Ajtkulov <ajtkulov(at)acm(dot)org>
To: pgsql-patches(at)postgresql(dot)org
Subject: strpos() && KMP
Date: 2007-08-01 20:18:22
Message-ID: 1167412014.20070802011822@acm.org
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Hello,

this patch allow to use Knuth-Morrison-Pratt algorithm for strpos() function
(see Cormen et al. Introduction to Algorithms, MIT Press, 2001).

It also works with multibyte wchar.

In worst case current brute force strpos() takes O(n * m) (n && m is length of strings)
time (example: 'aaa...aaab' search in 'aaa...aaa').
KMP algo always takes O(n + m) time.
To check this someone need to create a table with one text attribute, and insert several thousands
record 'aa..aa'(for example, with lenght = 1000) . After execute "select count(*) from test where
strpos(a, 'aaa....aab') > 0;" on current and modified version.

Also, I advise to use "select .. where strpos(att, 'word') > 0;" instead "select .. where attr like '%word%'"
(strpos must be faster than regex).

In general, this belongs to artificial expressions. In natural language KMP is equal (execution time)
current strpos() nearly.

----
Ajtkulov Pavel
ajtkulov(at)acm(dot)org

P. S. Sorry for prime English.

Attachment Content-Type Size
varlena.c.patch application/octet-stream 5.4 KB

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