Re: "like" and index

2009/2/25 Michael Monnerie <michael(dot)monnerie(at)is(dot)it-management(dot)at>

> 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;
>
> mfg zmi
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