Improve node type forward reference

Instead of using Node *, we can use an incomplete struct.  That way,
everything has the correct type and fewer casts are required.  This
technique is already used elsewhere in node type definitions.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/637eeea8-5663-460b-a114-39572c0f6c6e%40eisentraut.org

src/backend/commands/createas.c

@@ -133,7 +133,7 @@ create_ctas_internal(List *attrList, IntoClause *into)
 	if (is_matview)
 	{
 		/* StoreViewQuery scribbles on tree, so make a copy */
-		Query	   *query = (Query *) copyObject(into->viewQuery);
+		Query	   *query = copyObject(into->viewQuery);
 
 		StoreViewQuery(intoRelationAddr.objectId, query, false);
 		CommandCounterIncrement();

src/backend/parser/analyze.c

@@ -3077,7 +3077,7 @@ transformCreateTableAsStmt(ParseState *pstate, CreateTableAsStmt *stmt)
 		 * in the IntoClause because that's where intorel_startup() can
 		 * conveniently get it from.
 		 */
-		stmt->into->viewQuery = (Node *) copyObject(query);
+		stmt->into->viewQuery = copyObject(query);
 	}
 
 	/* represent the command as a utility Query */

src/include/nodes/primnodes.h

@@ -152,8 +152,8 @@ typedef struct TableFunc
  * For CREATE MATERIALIZED VIEW, viewQuery is the parsed-but-not-rewritten
  * SELECT Query for the view; otherwise it's NULL.  This is irrelevant in
  * the query jumbling as CreateTableAsStmt already includes a reference to
- * its own Query, so ignore it.  (Although it's actually Query*, we declare
+ * its own Query, so ignore it.  (We declare it as struct Query* to avoid a
- * it as Node* to avoid a forward reference.)
+ * forward reference.)
  */
 typedef struct IntoClause
 {
@@ -166,7 +166,7 @@ typedef struct IntoClause
 	OnCommitAction onCommit;	/* what do we do at COMMIT? */
 	char	   *tableSpaceName; /* table space to use, or NULL */
 	/* materialized view's SELECT query */
-	Node	   *viewQuery pg_node_attr(query_jumble_ignore);
+	struct Query *viewQuery pg_node_attr(query_jumble_ignore);
 	bool		skipData;		/* true for WITH NO DATA */
 } IntoClause;