I wrote:
> BTW, a different approach that might be worth considering is to say that
> the nodetree representation of one of these things *is* a FuncExpr, and
> the new magic thing is just that we invent a new CoercionForm value
> which causes ruleutils.c to print the expression as a subscripting op.
> I'm not quite convinced that that's a good idea --- a "coercion format"
> that says "subscript" seems a bit weird --- but it would greatly reduce
> the number of places you'd have to touch.

After sleeping on it, that approach is starting to sound better to me.
Consider a design like this:

* Leave ArrayRef strictly alone, and introduce new infrastructure beside
it for non-array containers.  That sounds ugly at first, but it has two
significant advantages: you don't have to refactor or even touch any
array-related code, and you do not have to worry about somebody objecting
to the patch because it adds unacceptable overhead to existing array
operations.  Converting array ops to go through a function-call API surely
must add *some* overhead, and at this point we don't have enough info to
be certain it would be negligibly small.  (Testing the existing patch
cannot prove that, since as I noted yesterday, you're missing a lot of
plan-time manipulations that need to happen for a generic function call.)

* Let the node representation for non-array-container access be FuncExpr
with new value(s) of funcformat.  You'd need to design the exact
representation to be used for the subscript arguments, but that doesn't
seem horribly complicated.  In this way, you're not on the hook to
duplicate all the node-processing infrastructure for either ArrayRef
or FuncExpr --- ideally, you won't need to do much more in the core
system than provide the parse-time callout and write suitable deparsing
logic in ruleutils.c.

The ugliness of thinking that "subscripting" is a kind of coercion could
be dealt with by changing funcformat to some new enum type named something
like, say, FuncDisplayForm.  This would require touching all the places
that mess with funcformat, but that could be a good thing anyway, because
you'd be sure that you'd looked at everything that might need changes.

                        regards, tom lane

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