Re: [HACKERS] TODO item: Implement Boyer-Moore searching (First time hacker)

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> After reading the wikipedia article on Boyer-Moore search algorithm, it
> looks to me like this patch actually implements the simpler
> Boyer-Moore-Horspool algorithm that only uses one lookup table. That's
> probably fine, as it ought to be faster on small needles and haystacks
> because it requires less effort to build the lookup tables, even though
> the worst-case performance is worse. It should still be faster than what
> we have now.

Hmm.  B-M-H has worst case search speed O(M*N) (where M = length of
pattern, N = length of search string); whereas full B-M is O(N).
Maybe we should build the second table when M is large?  Although
realistically that is probably gilding the lily, since frankly there
haven't been many real-world complaints about the speed of these
functions anyway ...

> The skip table really should be constructed only once in
> text_position_start and stored in TextPositionState. That would make a
> big difference to the performance of those functions that call
> text_position_next repeatedly: replace_text, split_text and text_to_array.

+1.  The heuristic about big a skip table to make may need some
adjustment as well, since it seems to be considering only a single
search.

            regards, tom lane