SELECT DISTINCT very slow - Mailing list pgsql-general

Hi,
Can anybody explain this:

Records: 600,000
Field Width: Approximately 20 UTF8 characters - type is VARCHAR(25)
Field is Indexed.

SELECT DISTINCT field FROM table;

Takes about 6 seconds. There are 111 distinct items.

On Sqlite, and another place where I have a B+Tree, this query is
faster than my eye can measure.

Is this a well known issue?

Thanks,
Ben