Prevent undistribute_table calls for partitions

src/backend/distributed/commands/create_distributed_table.c

@@ -1592,6 +1592,16 @@ UndistributeTable(Oid relationId)
 							   "because it is a foreign table.")));
 	}
 
+	if (PartitionTable(relationId))
+	{
+		Oid parentRelationId = PartitionParentOid(relationId);
+		char *parentRelationName = get_rel_name(parentRelationId);
+		ereport(ERROR, (errmsg("Cannot undistribute table "
+							   "because it is a partition."),
+						errhint("Undistribute the partitioned table \"%s\" instead.",
+								parentRelationName)));
+	}
+
 	List *preLoadCommands = GetPreLoadTableCreationCommands(relationId, true);
 	List *postLoadCommands = GetPostLoadTableCreationCommands(relationId);
 

src/test/regress/expected/undistribute_table.out

@@ -194,6 +194,11 @@ SELECT * FROM partitioned_table_6_10 ORDER BY 1, 2;
   7 | 2
 (1 row)
 
+-- undistributing partitions are not supported
+SELECT undistribute_table('partitioned_table_1_5');
+ERROR:  Cannot undistribute table because it is a partition.
+HINT:  Undistribute the partitioned table "partitioned_table" instead.
+-- we can undistribute partitioned parent tables
 SELECT undistribute_table('partitioned_table');
 NOTICE:  Undistributing the partitions of undistribute_table.partitioned_table
 NOTICE:  Creating a new local table for undistribute_table.partitioned_table_1_5

src/test/regress/sql/undistribute_table.sql

@@ -77,6 +77,9 @@ SELECT * FROM partitioned_table ORDER BY 1, 2;
 SELECT * FROM partitioned_table_1_5 ORDER BY 1, 2;
 SELECT * FROM partitioned_table_6_10 ORDER BY 1, 2;
 
+-- undistributing partitions are not supported
+SELECT undistribute_table('partitioned_table_1_5');
+-- we can undistribute partitioned parent tables
 SELECT undistribute_table('partitioned_table');
 
 SELECT logicalrelid FROM pg_dist_partition WHERE logicalrelid::regclass::text LIKE 'partitioned\_table%'  ORDER BY 1;