Allow dropping sequences on mx workers

This change allows users to drop sequences on MX workers. Previously, Citus didn't allow dropping
sequences on MX workers because it could cause shards to be dropped if `DROP SEQUENCE ... CASCADE`
is used. We now allow that since allowing sequence creation but not dropping hurts user experience
and also may cause problems with custom Citus solutions.

src/backend/distributed/master/master_delete_protocol.c

@@ -235,15 +235,14 @@ master_drop_sequences(PG_FUNCTION_ARGS)
 	Datum sequenceText = 0;
 	bool isNull = false;
 	StringInfo dropSeqCommand = makeStringInfo();
+	bool coordinator = IsCoordinator();
 
-	/* do nothing if DDL propagation is switched off */
-	if (!EnableDDLPropagation)
+	/* do nothing if DDL propagation is switched off or this is not the coordinator */
+	if (!EnableDDLPropagation || !coordinator)
 	{
 		PG_RETURN_VOID();
 	}
 
-	EnsureCoordinator();
-
 	/* iterate over sequence names to build single command to DROP them all */
 	sequenceIterator = array_create_iterator(sequenceNamesArray, 0, NULL);
 	while (array_iterate(sequenceIterator, &sequenceText, &isNull))

src/test/regress/expected/multi_unsupported_worker_operations.out

@@ -385,6 +385,36 @@ DELETE FROM pg_dist_shard_placement WHERE nodeport = :worker_2_port AND shardid
 SELECT master_get_new_placementid();
 ERROR:  operation is not allowed on this node
 HINT:  Connect to the coordinator and run it again.
+-- Show that sequences can be created and dropped on worker nodes
+CREATE TABLE some_table_with_sequence(a int, b BIGSERIAL, c BIGSERIAL);
+DROP TABLE some_table_with_sequence;
+CREATE SEQUENCE some_sequence;
+DROP SEQUENCE some_sequence;
+-- Show that dropping the sequence of an MX table with cascade harms the table and shards
+BEGIN;
+\d mx_table
+                           Table "public.mx_table"
+ Column |  Type   |                        Modifiers                         
+--------+---------+----------------------------------------------------------
+ col_1  | integer | 
+ col_2  | text    | 
+ col_3  | bigint  | not null default nextval('mx_table_col_3_seq'::regclass)
+
+DROP SEQUENCE mx_table_col_3_seq CASCADE;
+NOTICE:  drop cascades to 4 other objects
+DETAIL:  drop cascades to default for table mx_table_1270000 column col_3
+drop cascades to default for table mx_table_1270002 column col_3
+drop cascades to default for table mx_table_1270004 column col_3
+drop cascades to default for table mx_table column col_3
+\d mx_table
+   Table "public.mx_table"
+ Column |  Type   | Modifiers 
+--------+---------+-----------
+ col_1  | integer | 
+ col_2  | text    | 
+ col_3  | bigint  | not null
+
+ROLLBACK;
 -- Cleanup
 \c - - - :master_port
 DROP TABLE mx_table;

src/test/regress/sql/multi_unsupported_worker_operations.sql

@@ -204,6 +204,19 @@ DELETE FROM pg_dist_shard_placement WHERE nodeport = :worker_2_port AND shardid
 -- master_get_new_placementid
 SELECT master_get_new_placementid();
 
+-- Show that sequences can be created and dropped on worker nodes
+CREATE TABLE some_table_with_sequence(a int, b BIGSERIAL, c BIGSERIAL);
+DROP TABLE some_table_with_sequence;
+CREATE SEQUENCE some_sequence;
+DROP SEQUENCE some_sequence;
+
+-- Show that dropping the sequence of an MX table with cascade harms the table and shards
+BEGIN;
+\d mx_table
+DROP SEQUENCE mx_table_col_3_seq CASCADE;
+\d mx_table
+ROLLBACK;
+
 -- Cleanup
 \c - - - :master_port
 DROP TABLE mx_table;