Re: *_collapse_limit, geqo_threshold

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> I guess the question is whether there is anyone who has had a contrary
> experience.  (There must have been some benchmarks to justify adding
> geqo at some point?)

The CVS history shows that geqo was integrated on 1997-02-19, which
I think means that it must have been developed against Postgres95
(or even earlier Berkeley releases?).  That was certainly before any
of the current community's work on the optimizer began.  A quick look
at the code as it stood on that date suggests that the regular
optimizer's behavior for large numbers of rels was a lot worse than it
is today --- notably, it looks like it would consider a whole lot more
Cartesian-product joins than we do now; especially if you had "bushy"
mode turned on, which you'd probably have to do to find good plans in
complicated cases.  There were also a bunch of enormous inefficiencies
that we've whittled down over time, such as the mechanisms for comparing
pathkeys or the use of integer Lists to represent relid sets.

So while I don't doubt that geqo was absolutely essential when it was
written, it's fair to question whether it still provides a real win.
And we could definitely stand to take another look at the default
thresholds.
        regards, tom lane