> Why is this query using a seq scan rather than a index scan? 

Because it thinks a seq scan will be faster.

> i notice 
> the diff between the estimated rows and actual rows (almost 2000).

Yes, ANALYZE, and possibly increasing the column stats, should help that.

> Can this affect the query plan? i think this is a problem of
> statistics, am i right? if so, what can be done?

Well, if the estimate was accurate, PG would be even *more* likely to use a 
seq scan (more rows).

I think maybe you should establish whether a seq scan actually *is* faster?   
Perhaps do SET enable_seqscan = false and then re-run the query a few times?

Josh Berkus
Aglio Database Solutions
San Francisco

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