Re: query optimiser changes 6.5->7.0

Joseph Shraibman <jks@selectacast.net> writes:
> OK this may seem like a stupid question, but isn't index scan always
> better except for the pathalogical simple case where the work to be done
> is trivial anyway?

No.  If it were, the optimizer would be a whole lot simpler ;-)

In practice an indexscan only wins if it will visit a relatively
small percentage of the tuples in the table.  The $64 questions
are how small is small enough, and how can the optimizer guess
how many tuples will be hit in advance of doing the query...

			regards, tom lane