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Date: Thu, 18 Dec 2014 20:03:13 +0000 (UTC)
From: Kevin Grittner <kgrittn@ymail.com>
Reply-To: Kevin Grittner <kgrittn@ymail.com>
To: Giuseppe Broccolo <giuseppe.broccolo@2ndquadrant.it>,
 Robert DiFalco <robert.difalco@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
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 <123557937.571326.1418932993939.JavaMail.yahoo@jws100182.mail.ne1.yahoo.com>
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 <CAFzmHiXNZUU36WBPLSJY9gDfHrqSjBOZkNOLXHgS8ASV=4wZZw@mail.gmail.com>
References: 
 <CAFzmHiXNZUU36WBPLSJY9gDfHrqSjBOZkNOLXHgS8ASV=4wZZw@mail.gmail.com>
Subject: Re: Question about trigram GIST index
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Giuseppe Broccolo wrote:

> I'm not sure about the '%' operator, but I'm sure that the GIST
> index will never be used in the
>
>     SELECT * FROM users WHERE lower(name) LIKE '%john%';
>
> query; it is used for left or right anchored search, such as
> 'john%' or '%john'.
It *will* use a *trigram* index for a non-anchored search.

test=# create table words (word text not null);
CREATE TABLE
test=# copy words from '/usr/share/dict/words';
COPY 99171
test=# CREATE EXTENSION pg_trgm;
CREATE EXTENSION
test=# CREATE INDEX words_trgm ON words USING gist (word gist_trgm_ops);
CREATE INDEX
test=# vacuum analyze words;
VACUUM
test=# explain analyze select * from words where word like '%john%';
                                                                                                         QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on words  (cost=4.36..40.24 rows=10 width=9) (actual time=17.758..17.772 rows=8 loops=1)
   Recheck Cond: (word ~~ '%john%'::text)
   Rows Removed by Index Recheck: 16
   Heap Blocks: exact=4
   ->   Bitmap Index Scan on words_trgm  (cost=0.00..4.36 rows=10 width=0) (actual time=17.708..17.708 rows=24 loops=1)
         Index Cond: (word ~~ '%john%'::text)
Planning time: 0.227 ms
Execution time: 17.862 ms
(8 rows)

test=# explain analyze select * from words where word ilike '%john%';
                                                                                                             QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on words  (cost=44.05..556.57 rows=1002 width=9) (actual time=12.151..12.197 rows=24 loops=1)
   Recheck Cond: (word ~~* '%john%'::text)
   Heap Blocks: exact=4
   ->  Bitmap Index Scan on words_trgm  (cost=0.00..43.80 rows=1002 width=0) (actual time=12.124..12.124 rows=24 loops=1)
        Index Cond: (word ~~* '%john%'::text)
Planning time: 0.392 ms
Execution time: 12.252 ms
(7 rows)

Note that a trigram index is case-insensitive; doing a
case-sensitive search requires an extra Recheck node to eliminate
the rows that match in the case-insensitive index scan but have
different capitalization.  Because of that case-sensitive is
slower.

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


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