Re: making EXPLAIN extensible

> You only
> need the second hook if you want to check the values of options
> against the values of other options.
+1

I did not think of adding a new hook, because there must be a really good
reason to add a new hook. I think it's justified for this case. It's better than
my approach since the extension author can just put all their checks in one
place rather than having to register a bunch of handlers.

> some loadable module adds two new options LEFT and RIGHT and wants to
> check that you don't specify LEFT and RIGHT together? Either they
> register the same validate handler for both, or they register the real
> validate handler for one and a no-op handler for the other. Neither of
> those options seems very appealing.

When I thought about this, I figured that one of the options will register
a validate handler and the other option will set the handler to NULL. But,
I do see why this is not appealing.

--
Sami