Include values of A_Const nodes in query jumbling

Like the implementation for node copy, write and read, this node
requires a custom implementation so as the query jumbling is able to
consider the correct value assigned to it, depending on its type (int,
float, bool, string, bitstring).

Based on a dump of pg_stat_statements from the regression database, this
would confuse the query jumbling of the following queries:
- SET.
- COPY TO with SELECT queries.
- START TRANSACTION with different isolation levels.
- ALTER TABLE with default expressions.
- CREATE TABLE with partition bounds.

Note that there may be a long-term argument in tracking the location of
such nodes so as query strings holding such nodes could be normalized,
but this is left as a separate discussion.

Oversight in 3db72eb.

Discussion: https://postgr.es/m/Y9+HuYslMAP6yyPb@paquier.xyz

src/backend/nodes/queryjumblefuncs.c

@@ -49,6 +49,7 @@ static void AppendJumble(JumbleState *jstate,
 						 const unsigned char *item, Size size);
 static void RecordConstLocation(JumbleState *jstate, int location);
 static void _jumbleNode(JumbleState *jstate, Node *node);
+static void _jumbleA_Const(JumbleState *jstate, Node *node);
 static void _jumbleList(JumbleState *jstate, Node *node);
 static void _jumbleRangeTblEntry(JumbleState *jstate, Node *node);
 
@@ -313,6 +314,40 @@ _jumbleList(JumbleState *jstate, Node *node)
 	}
 }
 
+static void
+_jumbleA_Const(JumbleState *jstate, Node *node)
+{
+	A_Const    *expr = (A_Const *) node;
+
+	JUMBLE_FIELD(isnull);
+	if (!expr->isnull)
+	{
+		JUMBLE_FIELD(val.node.type);
+		switch (nodeTag(&expr->val))
+		{
+			case T_Integer:
+				JUMBLE_FIELD(val.ival.ival);
+				break;
+			case T_Float:
+				JUMBLE_STRING(val.fval.fval);
+				break;
+			case T_Boolean:
+				JUMBLE_FIELD(val.boolval.boolval);
+				break;
+			case T_String:
+				JUMBLE_STRING(val.sval.sval);
+				break;
+			case T_BitString:
+				JUMBLE_STRING(val.bsval.bsval);
+				break;
+			default:
+				elog(ERROR, "unrecognized node type: %d",
+					 (int) nodeTag(&expr->val));
+				break;
+		}
+	}
+}
+
 static void
 _jumbleRangeTblEntry(JumbleState *jstate, Node *node)
 {

src/include/nodes/parsenodes.h

@@ -355,7 +355,7 @@ union ValUnion
 
 typedef struct A_Const
 {
-	pg_node_attr(custom_copy_equal, custom_read_write)
+	pg_node_attr(custom_copy_equal, custom_read_write, custom_query_jumble)
 
 	NodeTag		type;
 	union ValUnion val;