Function pre_dispatch_function_evaluation_mutator

src/backend/cdb/cdbmutate.c:1521–1790

* Let's evaluate all STABLE functions that have constant args before * dispatch, so we get a consistent view across QEs * * Also, if this is a single_row insert, let's evaluate nextval() and * currval() before dispatching */

 * currval() before dispatching
 */
static Node *
pre_dispatch_function_evaluation_mutator(Node *node,
										 pre_dispatch_function_evaluation_context *context)
{
	Node	   *new_node = 0;

	if (node == NULL)
		return NULL;

	if (IsA(node, Param))
	{
		Param	   *param = (Param *) node;

		/* Not replaceable, so just copy the Param (no need to recurse) */
		return (Node *) copyObject(param);
	}
	else if (IsA(node, FuncExpr))
	{
		FuncExpr   *expr = (FuncExpr *) node;
		List	   *args;
		ListCell   *arg;
		Expr	   *simple;
		FuncExpr   *newexpr;
		bool		has_nonconst_input;

		Form_pg_proc funcform;
		EState	   *estate;
		ExprState  *exprstate;
		MemoryContext oldcontext;
		Datum		const_val;
		bool		const_is_null;
		int16		resultTypLen;
		bool		resultTypByVal;

		Oid			funcid;
		HeapTuple	func_tuple;

		/*
		 * Reduce constants in the FuncExpr's arguments. We know args is
		 * either NIL or a List node, so we can call expression_tree_mutator
		 * directly rather than recursing to self.
		 */
		args = (List *) expression_tree_mutator((Node *) expr->args,
												pre_dispatch_function_evaluation_mutator,
												(void *) context);

		funcid = expr->funcid;

		newexpr = makeNode(FuncExpr);
		newexpr->funcid = expr->funcid;
		newexpr->funcresulttype = expr->funcresulttype;
		newexpr->funcretset = expr->funcretset;
		newexpr->funcvariadic = expr->funcvariadic;
		newexpr->funcformat = expr->funcformat;
		newexpr->funccollid = expr->funccollid;
		newexpr->inputcollid = expr->inputcollid;
		newexpr->args = args;
		newexpr->location = expr->location;