Slow joins

I have a speed problem with a query that involves 3 tables.
I focused the problem in a scan in a big table (70000 rows) using a indexed
field. The field is a "flag" field which values only can be 'Y' or 'N'.
In the query I look for the values 'N' (average value = 10 rows over the
70000). Postgres does a Seq Scan.

How can I speed up that scan? How can I force the use of the index?

Where can I found general recomendations to speed up queries?

TIA

    Roberto

--
Roberto Moreda
Resp. Dpto. Informática Handem/San Luis
Tlf +34 981 779000
Fax +34 981 779022
Pol. Piadela Sur, Autovía A6 Sal.567
15300 Betanzos (A Coruña) - España