strpos() && KMP

[Pavel Ajtkulov <ajtkulov(at)acm(dot)org>]

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.

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