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Re: pg_trgm performance

From: "Guillaume Smet" <guillaume(dot)smet(at)gmail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: pg_trgm performance
Date: 2007-02-26 12:42:40
Message-ID: 1d4e0c10702260442m1fba9dc4r30ada1044f869a3b@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On 2/24/07, Steinar H. Gunderson <sgunderson(at)bigfoot(dot)com> wrote:

Thanks for your time.

> GiN version, short:
>    ->  Bitmap Heap Scan on tags  (cost=8.64..151.79 rows=41 width=0) (actual time=5.555..30.157 rows=7 loops=1)
>          Filter: (title % 'foo'::text)
>          ->  Bitmap Index Scan on trgm_idx  (cost=0.00..8.63 rows=41 width=0) (actual time=2.857..2.857 rows=5555 loops=1)
>                Index Cond: (title % 'foo'::text)

This is currently the worst case in the gist - gin comparison because
in the index scan, gin version doesn't have the length of the indexed
string. So it returns a lot of rows which have every trigram of your
search string but has in fact a low similarity due to the length of
the indexed string (5555 rows -> 7 rows).
It cannot be fixed at the moment due to the way GIN indexes work.

> So, the GiN version seems to be a bit faster for long queries, but it's still
> too slow -- in fact, _unindexed_ versions give 141ms, 342ms, 725ms for these
> three queries, so for the longer queries, the gain is only about a factor
> two. (By the way, I would like to stress that this is not my personal music
> collection! :-P)

The fact is that pg_trgm is designed to index words and not to index
long sentences. I'm not that surprised it's slow in your case.

It's also my case but following the instructions in README.pg_trgm I
created a dictionary of words using tsearch2 (stat function) and I use
pg_trgm on this dictionary to find similar words in my dictionary.

For example, I rewrite the search:
auberge cevenes
as:
(auberge | auberges | aubberge | auberg) & (ceven | cene | cevenol | cevennes)
using pg_trgm and my query can find Auberge des CĂ©vennes (currently
it's limited to the 4th most similar words but I can change it
easily).

--
Guillaume

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Subject: Re: pg_trgm performance
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