Nichlas =?iso-8859-1?Q?L=F6fdahl?= <[EMAIL PROTECTED]> writes:
> My question is, why doesn't the planner pick the same plan for Q1 & Q3?

I think it's mostly that after you've added and ANALYZEd the "age"
column, the planner has a pretty good idea of how many rows will pass
the "age > 17 AND age < 20" condition.  It can't do very much with the
equivalent condition in the original form, though, and in fact ends up
drastically underestimating the number of matching rows (86 vs reality
of 3021).  That leads directly to a bad plan choice :-(

                        regards, tom lane

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