Re: allowing extensions to control planner behavior

On Wed, 2024-08-28 at 16:35 -0400, Robert Haas wrote:
> On Wed, Aug 28, 2024 at 4:29 PM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> > Preserving a path for the right amount of time seems like the
> > primary
> > challenge for most of the use cases you raised (removing paths is
> > easier than resurrecting one that was pruned too early). If we try
> > to
> > keep a path around, that implies that we need to keep parent paths
> > around too, which leads to an explosion if we aren't careful.
> >
> > But we already solved all of that for pathkeys. We keep the paths
> > around if there's a reason to (a useful pathkey) and there's not
> > some
> > other cheaper path that also satisfies the same reason.
>
> But we've already solved it for this case, too. This is exactly what
> incrementing disabled_nodes does.

Hints are often described as something positive: use this index, use a
hash join here, etc. Trying to force a positive thing by adding
negative attributes to everything else is awkward. We've all had the
experience where we disable one plan type hoping for a good plan, and
we end up getting a different crazy plan that we didn't expect, and
need to disable a few more plan types.

Beyond awkwardness, one case where it matters is the interaction
between an extension that provides hints and an extension that offers a
CustomScan. How is the hints extension supposed to disable a path it
doesn't know about?

Regards,
Jeff Davis