Re: *_collapse_limit, geqo_threshold

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> I guess my point is that there's not a lot of obvious benefit in
> allowing the functionality to exist but handicapping it so that it's
> useful in as few cases as possible. If the consensus is that we want
> half a feature (but not more or less than half), that's OK with me,
> but it's not obvious to me why we should choose to want that.

Well, the question to my mind is whether the collapse_threshold GUCs in
their current form actually represent a feature ;-). They were put
in pretty much entirely on speculation that someone might find them
useful. Your argument is that they are not only useless but a foot-gun,
and so far we haven't got any clear contrary evidence. If we accept
that argument then we should take them out, not just change the default.

My own thought is that from_collapse_limit has more justification,
since it basically acts to stop a subquery from being flattened when
that would make the parent query too complex, and that seems like a
more understandable and justifiable behavior than treating JOIN
syntax specially. But I'm fine with removing join_collapse_limit
or reducing it to a boolean.

regards, tom lane