Re: Allowing ALTER TYPE to change storage strategy

I wrote:
> 3. Drop the ability for ALTER TYPE to promote from PLAIN to not-PLAIN
> typstorage, and adjust the typcache so that it only remembers boolean
> toastability not the specific toasting strategy.  Then the cache is
> still immutable so no need for update logic.
>
> I'm kind of liking #3, ugly as it sounds, because I'm not sure how
> much of a use-case there is for the upgrade-from-PLAIN case.
> Particularly now that TOAST is so ingrained in the system, it seems
> rather unlikely that a production-grade data type wouldn't have
> been designed to be toastable from the beginning, if there could be
> any advantage to that.  Either #1 or #2 seem like mighty high prices
> to pay for offering an option that might have no real-world uses.

Here's a v5 based on that approach.  I also added some comments about
the potential race conditions involved in recursing to domains.

			regards, tom lane