Re: "like" and index

[Tony Liao <tonyliao@xxxxxxxxxxxxx>]

2009/2/25 Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>

On Mittwoch 25 Februar 2009 Tony Liao wrote:
> 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||'%'

SELECT id FROM table_A WHERE prefix LIKE 'johnsmith%';

      in my table,the values of prefix are ('john','tom','anne','jim'......),so I don't think my SELECT was wrong.
       by the way,my postgres-server version is 8.36.thanks

> ,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.

Because your SELECT was wrong.

>       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.
>       thanks
>        ps:I have another table table_B would use table_B.prefix=
> table_A.prefix.so how can I create the index?

That's a foreign key, if I understand you correctly.

ALTER TABLE ONLY B
   ADD CONSTRAINT B_prefix_fkey FOREIGN KEY (prefix) REFERENCES
A(prefix) ON UPDATE CASCADE ON DELETE CASCADE;

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