SELECT LIKE 'xxx%' doesn't use index scan

db =# explain SELECT seq FROM tab WHERE str like 'edt%';
QUERY PLAN
-----------------------------------------------------------
Seq Scan on tab (cost=0.00..6465.55 rows=1 width=4)
Filter: (str ~~ 'edt%'::text)
(2 rows)

db=# explain SELECT seq FROM tab WHERE str = 'edta';
QUERY PLAN
------------------------------------------------------------------------------
Index Scan using tab_pkey on tab (cost=0.00..8.30 rows=1 width=4)
Index Cond: (str = 'edta'::text)
(2 rows)

PostgreSQL 8.2.5 on i586-mandriva-linux-gnu, compiled by GCC
i586-mandriva-linux-gnu-gcc (GCC) 4.2.2 20070909 (prerelease)
(4.2.2-0.RC.1mdv2008.0)

Thank you.

Edward Liu