Re: *_collapse_limit, geqo_threshold

Robert Haas <robertmhaas@gmail.com> writes:
> One possibility would be to remove join_collapse_limit entirely, but
> that would eliminate one possibily-useful piece of functionality that
> it current enables: namely, the ability to exactly specify the join
> order by setting join_collapse_limit to 1.  So one possibility would
> be to rename the variable something like explicit_join_order and make
> it a Boolean; another possibility would be to change the default value
> to INT_MAX.

As the person who put in those thresholds, I kind of prefer going over
to the boolean definition.  That was the alternative that we considered;
the numeric thresholds were used instead because they were easy to
implement and seemed to possibly offer more control.  But I'm not
convinced that anyone has really used them profitably.  I agree that
the ability to use JOIN syntax to specify the join order exactly (with
join_collapse_limit=1) is the only really solid use-case anyone has
proposed for either threshold.  I'm interested in Andreas' comment that
he has use-cases where using the collapse_limit is better than allowing
geqo to take over for very large problems ... but I think we need to see
those use-cases and see if there's a better fix.
        regards, tom lane