Optimizing pg_trgm makesign() (was Re: WIP: Fast GiST index build)

On 24.06.2011 11:40, Heikki Linnakangas wrote:
> On 21.06.2011 13:08, Alexander Korotkov wrote:
>> I believe it's due to relatively expensive penalty
>> method in that opclass.
>
> Hmm, I wonder if it could be optimized. I did a quick test, creating a
> gist_trgm_ops index on a list of English words from
> /usr/share/dict/words. oprofile shows that with the patch, 60% of the
> CPU time is spent in the makesign() function.

I couldn't resist looking into this, and came up with the attached
patch. I tested this with:

CREATE TABLE words (word text);
COPY words FROM '/usr/share/dict/words';
CREATE INDEX i_words ON words USING gist (word gist_trgm_ops);

And then ran "REINDEX INDEX i_words" a few times with and without the
patch. Without the patch, reindex takes about 4.7 seconds. With the
patch, 3.7 seconds. That's a worthwhile gain on its own, but becomes
even more important with Alexander's fast GiST build patch, which calls
the penalty function more.

I used the attached showsign-debug.patch to verify that the patched
makesign function produces the same results as the existing code. I
haven't tested the big-endian code, however.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com