Function simplify_function

src/backend/optimizer/util/clauses.c:4089–4183

* Subroutine for eval_const_expressions: try to simplify a function call * (which might originally have been an operator; we don't care) * * Inputs are the function OID, actual result type OID (which is needed for * polymorphic functions), result typmod, result collation, the input * collation to use for the function, the original argument list (not * const-simplified yet, unless process_arg

 * possible.
 */
static Expr *
simplify_function(Oid funcid, Oid result_type, int32 result_typmod,
				  Oid result_collid, Oid input_collid, List **args_p,
				  bool funcvariadic, bool process_args, bool allow_non_const,
				  eval_const_expressions_context *context)
{
	List	   *args = *args_p;
	HeapTuple	func_tuple;
	Form_pg_proc func_form;
	Expr	   *newexpr;

	/*
	 * We have three strategies for simplification: execute the function to
	 * deliver a constant result, use a transform function to generate a
	 * substitute node tree, or expand in-line the body of the function
	 * definition (which only works for simple SQL-language functions, but
	 * that is a common case).  Each case needs access to the function's
	 * pg_proc tuple, so fetch it just once.
	 *
	 * Note: the allow_non_const flag suppresses both the second and third
	 * strategies; so if !allow_non_const, simplify_function can only return a
	 * Const or NULL.  Argument-list rewriting happens anyway, though.
	 */
	func_tuple = SearchSysCache1(PROCOID, ObjectIdGetDatum(funcid));
	if (!HeapTupleIsValid(func_tuple))
		elog(ERROR, "cache lookup failed for function %u", funcid);
	func_form = (Form_pg_proc) GETSTRUCT(func_tuple);

	/*
	 * Process the function arguments, unless the caller did it already.
	 *
	 * Here we must deal with named or defaulted arguments, and then
	 * recursively apply eval_const_expressions to the whole argument list.
	 */
	if (process_args)
	{
		args = expand_function_arguments(args, false, result_type, func_tuple);
		args = (List *) expression_tree_mutator((Node *) args,
												eval_const_expressions_mutator,
												(void *) context);
		/* Argument processing done, give it back to the caller */
		*args_p = args;
	}

	/* Now attempt simplification of the function call proper. */

	newexpr = evaluate_function(funcid, result_type, result_typmod,
								result_collid, input_collid,
								args, funcvariadic,
								func_tuple, context);

	if (large_const(newexpr, context->max_size))
	{
		// folded expression prohibitively large
		newexpr = NULL;
	}

	if (!newexpr && allow_non_const && OidIsValid(func_form->prosupport))