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

On Mon, Oct 7, 2019 at 9:56 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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.

Typically, we support hook-sharing mostly by ad-hoc methods: when
installing a hook, you remember the previous value of the function
pointer, and arrange to call that function yourself. That's not a
great method. One problem with it is that you can't reasonably
uninstall a hook function, which would be a nice thing to be able to
do. We could do better by reusing the technique from on_shmem_exit or
RegisterXactCallbacks: keep an array of pointers, and call them in
order. I wish we'd retrofit all of our hooks to work more like that;
being able to unload shared libraries would be a good feature.

But 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.

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.

I wonder whether we might get a nicer solution to this problem if our
method of managing paths looked less something invented by a LISP
hacker, but I don't have a specific proposal in mind.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company