This question (and its variants) show quite often on #postgresql on IRC. People get sequential scans, and are worried that it's slow and bad.
So, I hope that this blogpost will shed some light on the subject why indexes are being chosen to be used, or not.
Robert Haas wrote, and Tom Lane committed (on 2nd of August) patch which adds interesting capability:
Log Message: ----------- Add ALTER TABLE ... ALTER COLUMN ... SET STATISTICS DISTINCT Robert Haas
Log message is rather terse, so what does it exactly do?
Continue reading Waiting for 8.5 – hinting for number of distinct values