Re: [v9.5] Custom Plan API

On Mon, Nov 10, 2014 at 6:55 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> I thought that in general if user has the API to register the custom path
> methods, it should have some way to unregister them and also user might
> need to register some different custom path methods after unregistering
> the previous one's. I think we should see what Robert or others have to
> say about this point before trying to provide such an API.

I wouldn't bother. As KaiGai says, if you want to shut the
functionality off, the provider itself can provide a GUC. Also, we
really have made no effort to ensure that loadable modules can be
safely unloaded, or hooked functions safely-unhooked.
ExecutorRun_hook is a good example. Typical of hook installation is
this:

prev_ExecutorRun = ExecutorRun_hook;
ExecutorRun_hook = pgss_ExecutorRun;

Well, if multiple extensions use this hook, then there's no hope of
unloading them exception in reverse order of installation. We
essentially end up creating a singly-linked list of hook users, but
with the next-pointers stored in arbitrarily-named, likely-static
variables owned by the individual extensions, so that nobody can
actually traverse it. This might be worth fixing as part of a
concerted campaign to make UNLOAD work, but unless somebody's really
going to do that I see little reason to hold this to a higher standard
than we apply elsewhere. The ability to remove extensions from this
hook won't be valuable by itself.

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