Function ExecEvalFunctionArgToConst

src/backend/executor/execExpr.c:4410–4450

* ExecEvalFunctionArgToConst * * Evaluates an argument of function expression and returns the result. * This is assumed to be used in the parser stage, where * dynamic evaluation such like Var is not available, though we put it * here so that we can extend it to be useful in other places later. */

 * here so that we can extend it to be useful in other places later.
 */
Datum
ExecEvalFunctionArgToConst(FuncExpr *fexpr, int argno, bool *isnull)
{
	Expr		   *aexpr;
	Oid				argtype;
	int32			argtypmod;
	Oid				argcollation;
	Const		   *result;

	/* argument number sanity check */
	if (argno < 0 || list_length(fexpr->args) <= argno)
		elog(ERROR, "invalid argument number found during evaluating function argument");

	aexpr = (Expr *) list_nth(fexpr->args, argno);
	/*
	 * Check if the expression can be evaluated in the Const fasion.
	 */
	if (ExecIsExprUnsafeToConst((Node *) aexpr))
		ereport(ERROR,
				(errcode(ERRCODE_DATA_EXCEPTION),
				 errmsg("unable to resolve function argument"),
				 errposition(exprLocation((Node *) aexpr))));

	argtype = exprType((Node *) aexpr);
	if (!OidIsValid(argtype))
		ereport(ERROR,
				(errcode(ERRCODE_INDETERMINATE_DATATYPE),
				 errmsg("unable to resolve function argument type"),
				 errposition(exprLocation((Node *) aexpr))));
	argtypmod = exprTypmod((Node *) aexpr);
	argcollation = exprCollation((Node *) aexpr);

	result = (Const *) evaluate_expr(aexpr, argtype, argtypmod, argcollation);
	/* evaluate_expr always returns Const */
	Assert(IsA(result, Const));

	if (isnull)
		*isnull = result->constisnull;

	return result->constvalue;
}