Re: The science of optimization in practical terms?

Robert Haas <robertmhaas@gmail.com> writes:
> I'm interested to know whether anyone else shares my belief that
> nested loops are the cause of most really bad plans.  What usually
> happens to me is that the planner develops some unwarranted optimism
> about the number of rows likely to be generated by the outer side of
> the join and decides that it's not worth sorting the inner side or
> building a hash table or using an index, and that the right thing to
> do is just rescan the inner node on every pass.  When the outer side
> returns three or four orders of magnitude more results than expected,
> ka-pow!

And then there is the other half of the world, who complain because it
*didn't* pick a nestloop for some query that would have run in much less
time if it had.
        regards, tom lane