Add a reverse-translation column number array to struct AppendRelInfo.

This provides for cheaper mapping of child columns back to parent
columns.  The one existing use-case in examine_simple_variable()
would hardly justify this by itself; but an upcoming bug fix will
make use of this array in a mainstream code path, and it seems
likely that we'll find other uses for it as we continue to build
out the partitioning infrastructure.

Discussion: https://postgr.es/m/12424.1575168015@sss.pgh.pa.us

src/backend/nodes/copyfuncs.c

@@ -2327,6 +2327,8 @@ _copyAppendRelInfo(const AppendRelInfo *from)
 	COPY_SCALAR_FIELD(parent_reltype);
 	COPY_SCALAR_FIELD(child_reltype);
 	COPY_NODE_FIELD(translated_vars);
+	COPY_SCALAR_FIELD(num_child_cols);
+	COPY_POINTER_FIELD(parent_colnos, from->num_child_cols * sizeof(AttrNumber));
 	COPY_SCALAR_FIELD(parent_reloid);
 
 	return newnode;

src/backend/nodes/equalfuncs.c

@@ -900,6 +900,8 @@ _equalAppendRelInfo(const AppendRelInfo *a, const AppendRelInfo *b)
 	COMPARE_SCALAR_FIELD(parent_reltype);
 	COMPARE_SCALAR_FIELD(child_reltype);
 	COMPARE_NODE_FIELD(translated_vars);
+	COMPARE_SCALAR_FIELD(num_child_cols);
+	COMPARE_POINTER_FIELD(parent_colnos, a->num_child_cols * sizeof(AttrNumber));
 	COMPARE_SCALAR_FIELD(parent_reloid);
 
 	return true;

src/backend/nodes/nodeFuncs.c

@@ -3103,6 +3103,7 @@ expression_tree_mutator(Node *node,
 
 				FLATCOPY(newnode, appinfo, AppendRelInfo);
 				MUTATE(newnode->translated_vars, appinfo->translated_vars, List *);
+				/* Assume nothing need be done with parent_colnos[] */
 				return (Node *) newnode;
 			}
 			break;

src/backend/nodes/outfuncs.c

@@ -2509,6 +2509,8 @@ _outAppendRelInfo(StringInfo str, const AppendRelInfo *node)
 	WRITE_OID_FIELD(parent_reltype);
 	WRITE_OID_FIELD(child_reltype);
 	WRITE_NODE_FIELD(translated_vars);
+	WRITE_INT_FIELD(num_child_cols);
+	WRITE_ATTRNUMBER_ARRAY(parent_colnos, node->num_child_cols);
 	WRITE_OID_FIELD(parent_reloid);
 }