Re: Fastest Index/Algorithm to find similar sentences

From: Kevin Grittner <kgrittn(at)ymail(dot)com>
To: Janek Sendrowski <janek12(at)web(dot)de>, "pgsql-general(at)postgresql(dot)org" <pgsql-general(at)postgresql(dot)org>
Subject: Re: Fastest Index/Algorithm to find similar sentences
Date: 2013-08-02 15:25:12
Message-ID: 1375457112.43393.YahooMailNeo@web162905.mail.bf1.yahoo.com
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Janek Sendrowski <janek12(at)web(dot)de> wrote:

> I also tried pg_trgm module, which works with tri-grams, but it's
> also very slow with 100.000+ rows.

Hmm.  I found the pg_trgm module very fast for name searches with
millions of rows *as long as I used KNN-GiST techniques*.  Were you
careful to do so?  Check out the "Index Support" section of this
page:

http://www.postgresql.org/docs/current/static/pgtrgm.html

While I have not tested this technique with a column containing
sentences, I would expect it to work well.  As a quick
confirmation, I imported the text form of War and Peace into a
table, with one row per *line* (because that was easier than
parsing sentence boundaries for a quick test).  That was over
65,000 rows.

test=# select * from war_and_peace order by linetext <-> 'young wealthy gay gentlemen' limit 3;
 lineno |                               linetext                                
--------+-----------------------------------------------------------------------
   9082 | The gentlemen assembled at Bilibin's were young, wealthy, gay society
  36575 | "Gentlemen, you are crushing me!..."
  55997 | * "Good day, gentlemen."
(3 rows)

test=# explain analyze select * from war_and_peace order by linetext <-> 'young wealthy gay gentlemen' limit 3;
                                                               QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.28..0.80 rows=3 width=53) (actual time=37.986..38.002 rows=3 loops=1)
   ->  Index Scan using wap_text on war_and_peace  (cost=0.28..11216.42 rows=65007 width=53) (actual time=37.984..37.999 rows=3 loops=1)
         Order By: (linetext <-> 'young wealthy gay gentlemen'::text)
 Total runtime: 38.180 ms
(4 rows)

To me, 38 milliseconds to search War and Peace for best matches to
a text string seems reasonable; I'm not sure what you're looking
for, since you didn't give any numbers.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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