Re: [HACKERS] [PATCH] Generic type subscripting

Dmitry Dolgov <9erthalion6@gmail.com> writes:
>> On Mon, Nov 30, 2020 at 02:26:19PM +0100, Dmitry Dolgov wrote:
>>> On Mon, Nov 30, 2020 at 04:12:29PM +0300, Alexander Korotkov wrote:
>>> The idea of an opaque field in SubscriptingRef structure is more
>>> attractive to me.  Could you please implement it?

>> Sure, doesn't seem to be that much work.

I just happened to notice this bit.  This idea is a complete nonstarter.
You cannot have an "opaque" field in a parsetree node, because then the
backend/nodes code has no idea what to do with it for
copy/compare/outfuncs/readfuncs.  The patch seems to be of the opinion
that "do nothing" is adequate, which it completely isn't.

Perhaps this is a good juncture at which to remind people that parse
tree nodes are read-only so far as the executor is concerned, so
storing something there only at execution time won't work either.

            regards, tom lane