Re: How to retain lesser paths at add_path()?

Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Oct 7, 2019 at 9:56 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> We could imagine, maybe, that a hook for the purpose of allowing an
>> additional dimension to be considered would be essentially a path
>> comparison function, returning -1, +1, or 0 depending on whether
>> path A is dominated by path B (on this new dimension), dominates
>> path B, or neither.  However, I do not see how multiple extensions
>> could usefully share use of such a hook.

> ... if we want to stick with the ad-hoc method, we could also just
> have four possible return values: dominates, dominated-by, both, or
> neither.

Right, and then *each* user of the hook would have to be prepared
to merge its result with the result from the previous user(s),
which is a complicated bit of logic that somebody would surely
get wrong, especially if (a) there's no prototype to copy from
and (b) testing only their own extension would not exercise it.

[ thinks a bit... ]  Maybe that could be improved if we can express
the result as a bitmask, defined in such a way that OR'ing (or maybe
AND'ing? haven't worked it out) the results from different comparisons
does the right thing.

> Still, this doesn't feel like very scalable paradigm, because this
> code gets called a lot.  Unless both calling the hook functions and
> the hook functions themselves are dirt-cheap, it's going to hurt, and
> TBH, I wonder if even the cost of detecting that the hook is unused
> might be material.

Yeah, I'm worried about that too.  This is quite a hot code path,
and so I don't think we can just assume that changes are free.
Still, if we could come up with a cleaner paradigm, maybe we could
buy back a few cycles in the core-code comparison logic, and thus
not come out behind from adding a hook.

            regards, tom lane