Tom Lane wrote:

> When benchmarking with data sets considerably larger than available
> buffer cache, I rather doubt that small random_page_cost would be a 
> good idea.  Still, you might as well experiment to see.

>From experience, I know the difference in response time can be huge when postgres 
chooses a sequential scan over an index scan.  In practice, do people experience as 
great a
difference when postgres incorrectly chooses an index scan over a sequential scan?  My 
is that the speed difference is a lot less for incorrectly choosing an index scan.  If 
this is the
case, it would be safer to chose a small value for random_page_cost. 

George Essig

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TIP 9: the planner will ignore your desire to choose an index scan if your
      joining column's datatypes do not match

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