On second thought, disable parallelism for prepared statements.

CREATE TABLE .. AS EXECUTE can turn an apparently read-only query into
a write operation, which parallel query can't handle.  It's a bit of a
shame that requires us to avoid parallel query for queries prepared via
PREPARE in all cases, but for right now it does.

src/backend/commands/prepare.c

@@ -159,7 +159,7 @@ PrepareQuery(PrepareStmt *stmt, const char *queryString)
 					   nargs,
 					   NULL,
 					   NULL,
-					   CURSOR_OPT_PARALLEL_OK,	/* allow parallel mode */
+					   0,		/* default cursor options */
 					   true);	/* fixed result */
 
 	/*