Two questions about join performance

Sorry for being so reiterative :)

The planifier isn't perfect for many queries...
Is there any way (without touch C code) to tell explicitly the plan of a query
to Postgres?

How can I manage the problem of select a few rows with a boolean atribute
when they are 5 rows in a table of 100000 rows?
I't must to be an index, but
the optimizer asumes that a Seq Scan is more cheap... yes, yes... I know :
if I ask for the 100000 then Seq Scan is the solution, but the interesting
query is the other : to extract the 5 rows with flag='Y' from whitin the
100000 rows with the flag='N'.

I'm in a big trouble because of all this.

TIA again.

Roberto

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Roberto Moreda
Resp. Dpto. Informática Handem/San Luis
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15300 Betanzos (A Coruña) - España