Optimizer Question

Hi All!

I've tried some developements with PG and was surprised
by optimizer... So:

1. Why in query: "select distinct Attr from Table;"
optimizer never use Attr's index and always pass by Seq Scan?

2. Why in general any queries like:
select Attr from Table where --any conditions with Attr--;
optimizer choose Seq Scan and not Attr's index, when using of
Attr's index significally increases performance...

Thanks for any help.
(dim)

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Dimitri KRAVTCHUK (dim) Sun Microsystems
Benchmark Engineer France
dimitri(at)France(dot)Sun(dot)COM
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