Re: broken join optimization? (8.0)

Michael Fuhr <mike(at)fuhr(dot)org> writes:
> On Tue, Oct 25, 2005 at 04:56:11PM -0700, chester c young wrote:
>> in php (for example) it's frequently nice to get the structure of a
>> table without any data,

> Have you considered "SELECT * FROM mytable LIMIT 0"?

Indeed.

> I see the same behavior in the latest 8.1beta code. Maybe one of
> the developers will comment on whether optimizing that is a simple
> change, a difficult change, not worth changing because few people
> find a use for it, or a behavior that can't be changed because of
> something we're not considering.

Not worth changing --- why should we expend cycles (even if it only
takes a few, which isn't clear to me offhand) on every join query, to
detect what's simply a brain-dead way of finding out table structure?
I can't think of any realistic scenarios for a constant-false join
clause.

The relevant bit of code is in initsplan.c:

/*
* If the clause is variable-free, we force it to be evaluated at its
* original syntactic level. Note that this should not happen for
* top-level clauses, because query_planner() special-cases them. But it
* will happen for variable-free JOIN/ON clauses. We don't have to be
* real smart about such a case, we just have to be correct.
*/
if (bms_is_empty(relids))
relids = qualscope;

Possibly you could get the planner to generate a gating Result node for
such a case, the way it does for constant-false top level WHERE clauses,
but I really doubt it's worth any extra cycles at all to make this
happen. The proposed example is quite unconvincing ... why would anyone
want to depend on the existence of a "dual" table rather than LIMIT 0?

regards, tom lane