ExecMakeTableFunctionResult vs. pre-evaluated functions

I've spent today messing with making the planner substitute inline
definitions of simple SQL functions, per the comment in
src/backend/optimizer/util/clauses.c:

* XXX Possible future improvement: if the func is SQL-language, and its
* definition is simply "SELECT expression", we could parse and substitute
* the expression here. This would avoid much runtime overhead, and perhaps
* expose opportunities for constant-folding within the expression even if
* not all the func's input args are constants. It'd be appropriate to do
* that here, not in the parser, since we wouldn't want it to happen until
* after rule substitution/rewriting.

It seems to work 99%, but I'm seeing this failure in the regression
tests:

CREATE FUNCTION getfoo(int) RETURNS int AS 'SELECT $1;' LANGUAGE SQL;
SELECT * FROM getfoo(1) AS t1;
! ERROR: ExecMakeTableFunctionResult: expression is not a function call

which of course happens because the table-function expression has been
reduced to just a constant "1" by the time the executor sees it.

A grotty answer is to not apply constant-expression folding to table
function RTE entries. A better answer would be to make
ExecMakeTableFunctionResult more flexible, but I'm not quite sure what
it should do if presented a non-function-call expression tree. Any
thoughts?

regards, tom lane

PS: another little problem is
regression=# explain SELECT * FROM getfoo(1) AS t1;
server closed the connection unexpectedly
but I'm sure that's just a lack of flexibility in explain.c ...