Re: Early WIP/PoC for inlining CTEs

I wrote:
> Therefore, I'm reversing my previous opinion that we should not have
> an explicit NOT MATERIALIZED option. I think we should add that, and
> the behavior ought to be:
> * No option given: inline if there's exactly one reference.
> * With MATERIALIZED: never inline.
> * With NOT MATERIALIZED: inline regardless of the number of references.
> (Obviously, we should not inline if there's RECURSIVE or the CTE
> potentially has side-effects, regardless of the user option;
> I don't think those cases are up for debate.)

Hearing no immediate pushback on that proposal, I went ahead and made
a version of the patch that does it like that, as attached. I also took
a stab at documenting it fully.

I was interested to find, while writing the docs, that it's a real
struggle to invent plausible reasons to write MATERIALIZED given the
above specification. You pretty much have to have lied to the planner,
eg by making a volatile function that's not marked volatile, before
there's a real need for that. Am I missing something? If I'm not,
then we're in a really good place backwards-compatibility-wise,
because the new default behavior shouldn't break any cases where people
weren't cheating.

regards, tom lane