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.
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.
David Rowley wrote:
I've done some more revisions to the patch. This has mostly just involved
tuning the skip table size based on the size of the search. This has
basically involved lots of benchmarks with different strings and calculating
the best size of table to use. The reason for this is to maintain fast
searches for smaller strings. The overhead of initialising a 256 element
array would probably out weigh the cost of the search if this were not done.
The size of the skip table increases with longer strings, or rather the size
that is utilised.
For smaller searches performance of the patch and existing version are very
similar. The patched version starts to out perform the existing version when
the needle and haystack become larger.
The patch wins hands down with searches that leads the existing function in
to dead ends, for example:
SELECT STRPOS('A AA AAA AAAA AAAAA AAAAAA AAAAAAA','AAAAAAA');
When searching for very small strings, say just a single character in a
sentence the existing function marginally beats the patched version.
Outside of Postgres I've done benchmarks where I've searched for every
combination of the search string in the search string. Like:
test | t |
test | te |
test | tes |
test | test |
test | e |
test | es |
test | est |
test | s |
test | st |
test | t |
I felt this was fair for both versions. The patched version beat the
unpatched version. The test I carried out was a string of 934 characters.
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