Re: "like" and index

In article <8f750b7c0902250259w6065515as350aca3b5d7d8173(at)mail(dot)gmail(dot)com>,
Tony Liao <tonyliao(at)yuehetone(dot)com> writes:

> hi all,
>       I have a table table_A (id serial,prefix varchar),for example.
>       now I want to get the id of "johnsmith"'s prefix match table_A.prefix,so
> I do select id from table_A where 'johnsmith' like prefix||'%'  ,the table_A is
> very large so I would like to make index. create table_A_index on table_A
> (prefix)
>       I try to explain analyze,but it doesn't work ,it use seq scan.
>       I try another index. drop index table_A_index; create table_A_index on
> table_A(prefix varchar_pattern_ops); it doesn't work,too.

If I understand you correctly, the "prefix" contrib package is what
you need:

CREATE TABLE tableA (
id serial NOT NULL,
prefix prefix_range NOT NULL,
PRIMARY KEY (id)
);

CREATE INDEX tableA_prefix_ix on tableA
USING gist (prefix gist_prefix_range_ops);

COPY tableA (prefix) FROM stdin;
john
tom
anne
jim
\.

INSERT INTO tableA (prefix)
SELECT x || 'test'
FROM generate_series (1, 10000) g(x);

ANALYZE tableA;

EXPLAIN ANALYZE
SELECT id, prefix
FROM tableA
WHERE prefix @> 'johnsmith';

will return something like that:

Bitmap Heap Scan on tablea (cost=4.33..32.10 rows=10 width=19) (actual time=0.035..0.036 rows=1 loops=1)
Recheck Cond: (prefix @> 'johnsmith[]'::prefix_range)
-> Bitmap Index Scan on tablea_prefix_ix (cost=0.00..4.33 rows=10 width=0) (actual time=0.026..0.026 rows=1 loops=1)
Index Cond: (prefix @> 'johnsmith[]'::prefix_range)
Total runtime: 0.133 ms