"like" and index

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

yours, Tony