> Is it possible to use an index for like queries on a citext column?
> I'm using pg 8.4.1 on windows - with no changes to the default configuration.
>
> For example:
>
> CREATE TABLE test ( citext citext NOT NULL );
> INSERT INTO test select md5(random()::text) FROM generate_series(0, 1000000, 1);
> CREATE INDEX test_citext_idx ON test USING btree(citext);
> vacuum analyze test;
>
> explain analyze select * from test where citext like '5555%'
> ...
> Seq Scan on test (cost=0.00..20834.03 rows=5000 width=33)
> (actual time=45.916..3691.540 rows=16 loops=1)
> Filter: (citext ~~ '5555%'::citext)
> Total runtime: 3691.676 ms
>
> set enable_seqscan = off;
> explain analyze select * from test where citext like '5555%'
> ...
> Seq Scan on test (cost=10000000000.00..10000020834.03 rows=5000 width=33)
> (actual time=45.578..3761.687 rows=16 loops=1)
> Filter: (citext ~~ '5555%'::citext)
> Total runtime: 3761.860 ms
>
>
> With equal I'm getting an index scan
> explain analyze select * from test where citext =
> '55559cb65689f035766eb69ed615afd4'
> Index Scan using test_citext_idx on test (cost=0.00..8.56 rows=1 width=33)
> (actual time=0.452..0.462 rows=1 loops=1)
> Index Cond: (citext = '55559cb65689f035766eb69ed615afd4'::citext)
> Total runtime: 0.558 ms
>
> So, is there any way to get the like queries to use the index?
>
>
I don't know if it is a good practice (in this case), but you can
create an index per value
(expressional indexes).
CREATE INDEX xx ON table (citext_column ) WHERE citext_column ~~ '5555%';
But IMHO if you are interest only in the firsts values (example 4)
you can create an index using hash_text function:
CREATE INDEX xx ON table (hashtext(substring(citext_col,1,4)));
--disable seqscan for little tables
explain select * from pp where (hashtext(substring(i,1,4))) = hashtext('0.06');
Without hashtext:
CREATE INDEX xx ON table (substring(citext_col,1,4));
explain select * from pp where substring(i,1,4) = '0.06';
The entire field to search:
CREATE INDEX xx ON table (hashtext(citext_col));
explain select * from pp where hashtext(i) = hashtext('all the field here');
It is useful?
--
Emanuel Calvo Franco
DBA at: www.siu.edu.ar
www.emanuelcalvofranco.com.ar