LIKE query on indexes

hi,
i have btree index on a text type field. i want see rows which starts with certain characters on that field. so i write a query like this:

SELECT * FROM mytable WHERE myfield LIKE 'john%'

since this condition is from start of the field, query planner should use index to find such elements but explain command shows me it will do a sequential scan.

is this lack of a feature or i am wrong somewhere?