Allow creating distributed tables in sequential mode

With https://github.com/citusdata/citus/pull/2780, we allow
COPY to use any number of connections that the executor used
in a tx block.

Meaning that, while COPYing data to the shards, create_distributed_table
could allow sequential mode.

src/backend/distributed/commands/create_distributed_table.c

@@ -1539,19 +1539,7 @@ CanUseExclusiveConnections(Oid relationId, bool localTableEmpty)
 	bool shouldRunSequential = MultiShardConnectionType == SEQUENTIAL_CONNECTION ||
 							   hasForeignKeyToReferenceTable;
 
-	if (!localTableEmpty && shouldRunSequential)
-	{
-		char *relationName = get_rel_name(relationId);
-
-		ereport(ERROR, (errmsg("cannot distribute \"%s\" in sequential mode "
-							   "because it is not empty", relationName),
-						errhint("If you have manually set "
-								"citus.multi_shard_modify_mode to 'sequential', "
-								"try with 'parallel' option. If that is not the "
-								"case, try distributing local tables when they "
-								"are empty.")));
-	}
-	else if (shouldRunSequential && ParallelQueryExecutedInTransaction())
+	if (shouldRunSequential && ParallelQueryExecutedInTransaction())
 	{
 		/*
 		 * We decided to use sequential execution. It's either because relation

src/test/regress/expected/foreign_key_restriction_enforcement.out

@@ -1282,8 +1282,8 @@ ERROR:  current transaction is aborted, commands ignored until end of transactio
 	DROP TABLE test_table_1 CASCADE;
 ERROR:  current transaction is aborted, commands ignored until end of transaction block
 ROLLBACK;
--- make sure that we cannot create hash distributed tables with
--- foreign keys to reference tables when they have data in it
+-- make sure that we can create hash distributed tables with
+-- even when foreign keys to reference tables and they have data in it
 BEGIN;
 	CREATE TABLE test_table_1(id int PRIMARY KEY);
 	INSERT INTO test_table_1 SELECT i FROM generate_series(0,100) i;
@@ -1300,16 +1300,21 @@ HINT:  To remove the local data, run: SELECT truncate_local_data_after_distribut
 (1 row)
 
 	SELECT create_distributed_table('test_table_2', 'id');
-ERROR:  cannot distribute "test_table_2" in sequential mode because it is not empty
-HINT:  If you have manually set citus.multi_shard_modify_mode to 'sequential', try with 'parallel' option. If that is not the case, try distributing local tables when they are empty.
+NOTICE:  Copying data from local table...
+NOTICE:  copying the data has completed
+DETAIL:  The local data in the table is no longer visible, but is still on disk.
+HINT:  To remove the local data, run: SELECT truncate_local_data_after_distributing_table($$test_fkey_to_ref_in_tx.test_table_2$$)
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
 	-- make sure that the output isn't too verbose
  	SET LOCAL client_min_messages TO ERROR;
-ERROR:  current transaction is aborted, commands ignored until end of transaction block
 	DROP TABLE test_table_2, test_table_1;
-ERROR:  current transaction is aborted, commands ignored until end of transaction block
 COMMIT;
--- the same test with above in sequential mode would still not work
--- since COPY cannot be executed in sequential mode
+-- the same test with above in sequential mode would just work
+-- as COPY can be executed in sequential mode
 BEGIN;
 	SET LOCAL citus.multi_shard_modify_mode TO 'sequential';
 	CREATE TABLE test_table_1(id int PRIMARY KEY);
@@ -1327,13 +1332,18 @@ HINT:  To remove the local data, run: SELECT truncate_local_data_after_distribut
 (1 row)
 
 	SELECT create_distributed_table('test_table_2', 'id');
-ERROR:  cannot distribute "test_table_2" in sequential mode because it is not empty
-HINT:  If you have manually set citus.multi_shard_modify_mode to 'sequential', try with 'parallel' option. If that is not the case, try distributing local tables when they are empty.
+NOTICE:  Copying data from local table...
+NOTICE:  copying the data has completed
+DETAIL:  The local data in the table is no longer visible, but is still on disk.
+HINT:  To remove the local data, run: SELECT truncate_local_data_after_distributing_table($$test_fkey_to_ref_in_tx.test_table_2$$)
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
 	-- make sure that the output isn't too verbose
  	SET LOCAL client_min_messages TO ERROR;
-ERROR:  current transaction is aborted, commands ignored until end of transaction block
 	DROP TABLE test_table_2, test_table_1;
-ERROR:  current transaction is aborted, commands ignored until end of transaction block
 COMMIT;
 -- we should be able to execute and DML/DDL/SELECT after we've
 -- switched to sequential via create_distributed_table

src/test/regress/expected/multi_truncate.out

@@ -501,24 +501,23 @@ INSERT INTO dist SELECT x,x FROM generate_series(1,10000) x;
 SELECT truncate_local_data_after_distributing_table('ref');
 ERROR:  cannot truncate a table referenced in a foreign key constraint by a local table
 DETAIL:  Table "dist" references "ref"
--- test that we do not allow distributing tables that have foreign keys to reference tables
+-- test that we allow distributing tables that have foreign keys to reference tables
 SELECT create_distributed_table('dist','id');
-ERROR:  cannot distribute "dist" in sequential mode because it is not empty
-HINT:  If you have manually set citus.multi_shard_modify_mode to 'sequential', try with 'parallel' option. If that is not the case, try distributing local tables when they are empty.
+NOTICE:  Copying data from local table...
+NOTICE:  copying the data has completed
+DETAIL:  The local data in the table is no longer visible, but is still on disk.
+HINT:  To remove the local data, run: SELECT truncate_local_data_after_distributing_table($$multi_truncate.dist$$)
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
 SHOW citus.multi_shard_modify_mode;
  citus.multi_shard_modify_mode
 ---------------------------------------------------------------------
  parallel
 (1 row)
 
--- distribute the table after a truncate
-TRUNCATE dist;
-SELECT create_distributed_table('dist','id');
- create_distributed_table
----------------------------------------------------------------------
-
-(1 row)
-
 -- the following should truncate ref and dist
 BEGIN;
 SELECT truncate_local_data_after_distributing_table('ref');

src/test/regress/expected/sequential_modifications.out

@@ -610,20 +610,63 @@ SELECT distributed_2PCs_are_equal_to_worker_count();
 (1 row)
 
 DROP TABLE test_seq_ddl_index;
--- create_distributed_table should fail on relations with data in sequential mode in and out transaction block
+-- create_distributed_table should works on relations with data in sequential mode in and out transaction block
 CREATE TABLE test_create_seq_table (a int);
 INSERT INTO test_create_seq_table VALUES (1);
 SET citus.multi_shard_modify_mode TO 'sequential';
 SELECT create_distributed_table('test_create_seq_table' ,'a');
-ERROR:  cannot distribute "test_create_seq_table" in sequential mode because it is not empty
-HINT:  If you have manually set citus.multi_shard_modify_mode to 'sequential', try with 'parallel' option. If that is not the case, try distributing local tables when they are empty.
+NOTICE:  Copying data from local table...
+NOTICE:  copying the data has completed
+DETAIL:  The local data in the table is no longer visible, but is still on disk.
+HINT:  To remove the local data, run: SELECT truncate_local_data_after_distributing_table($$test_seq_ddl.test_create_seq_table$$)
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+SELECT undistribute_table('test_create_seq_table');
+NOTICE:  creating a new table for test_seq_ddl.test_create_seq_table
+NOTICE:  moving the data of test_seq_ddl.test_create_seq_table
+NOTICE:  dropping the old test_seq_ddl.test_create_seq_table
+NOTICE:  renaming the new table to test_seq_ddl.test_create_seq_table
+ undistribute_table
+---------------------------------------------------------------------
+
+(1 row)
+
 RESET citus.multi_shard_modify_mode;
 BEGIN;
     SET LOCAL citus.multi_shard_modify_mode TO 'sequential';
     select create_distributed_table('test_create_seq_table' ,'a');
-ERROR:  cannot distribute "test_create_seq_table" in sequential mode because it is not empty
-HINT:  If you have manually set citus.multi_shard_modify_mode to 'sequential', try with 'parallel' option. If that is not the case, try distributing local tables when they are empty.
+NOTICE:  Copying data from local table...
+NOTICE:  copying the data has completed
+DETAIL:  The local data in the table is no longer visible, but is still on disk.
+HINT:  To remove the local data, run: SELECT truncate_local_data_after_distributing_table($$test_seq_ddl.test_create_seq_table$$)
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
 ROLLBACK;
+-- trigger switch-over when using single connection per worker
+BEGIN;
+SET citus.next_shard_id TO 16900;
+SET LOCAL citus.shard_count TO 4;
+SET LOCAL citus.multi_shard_modify_mode TO 'sequential';
+CREATE UNLOGGED TABLE trigger_switchover(a int, b int, c int, d int, e int, f int, g int, h int);
+INSERT INTO trigger_switchover
+  SELECT s AS a, s AS b, s AS c, s AS d, s AS e, s AS f, s AS g, s AS h FROM generate_series(1,250000) s;
+SELECT create_distributed_table('trigger_switchover','a');
+NOTICE:  Copying data from local table...
+NOTICE:  copying the data has completed
+DETAIL:  The local data in the table is no longer visible, but is still on disk.
+HINT:  To remove the local data, run: SELECT truncate_local_data_after_distributing_table($$test_seq_ddl.trigger_switchover$$)
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+ABORT;
 SET search_path TO 'public';
 DROP SCHEMA test_seq_ddl CASCADE;
 NOTICE:  drop cascades to 11 other objects

src/test/regress/sql/foreign_key_restriction_enforcement.sql

@@ -803,8 +803,8 @@ BEGIN;
 	DROP TABLE test_table_1 CASCADE;
 ROLLBACK;
 
--- make sure that we cannot create hash distributed tables with
--- foreign keys to reference tables when they have data in it
+-- make sure that we can create hash distributed tables with
+-- even when foreign keys to reference tables and they have data in it
 BEGIN;
 
 	CREATE TABLE test_table_1(id int PRIMARY KEY);
@@ -822,8 +822,8 @@ BEGIN;
 COMMIT;
 
 
--- the same test with above in sequential mode would still not work
--- since COPY cannot be executed in sequential mode
+-- the same test with above in sequential mode would just work
+-- as COPY can be executed in sequential mode
 BEGIN;
 
 	SET LOCAL citus.multi_shard_modify_mode TO 'sequential';

src/test/regress/sql/multi_truncate.sql

@@ -295,14 +295,10 @@ INSERT INTO dist SELECT x,x FROM generate_series(1,10000) x;
 -- test that we do not cascade truncates to local referencing tables
 SELECT truncate_local_data_after_distributing_table('ref');
 
--- test that we do not allow distributing tables that have foreign keys to reference tables
+-- test that we allow distributing tables that have foreign keys to reference tables
 SELECT create_distributed_table('dist','id');
 SHOW citus.multi_shard_modify_mode;
 
--- distribute the table after a truncate
-TRUNCATE dist;
-SELECT create_distributed_table('dist','id');
-
 -- the following should truncate ref and dist
 BEGIN;
 SELECT truncate_local_data_after_distributing_table('ref');

src/test/regress/sql/sequential_modifications.sql

@@ -326,12 +326,13 @@ COMMIT;
 SELECT distributed_2PCs_are_equal_to_worker_count();
 DROP TABLE test_seq_ddl_index;
 
--- create_distributed_table should fail on relations with data in sequential mode in and out transaction block
+-- create_distributed_table should works on relations with data in sequential mode in and out transaction block
 CREATE TABLE test_create_seq_table (a int);
 INSERT INTO test_create_seq_table VALUES (1);
 
 SET citus.multi_shard_modify_mode TO 'sequential';
 SELECT create_distributed_table('test_create_seq_table' ,'a');
+SELECT undistribute_table('test_create_seq_table');
 
 RESET citus.multi_shard_modify_mode;
 
@@ -340,5 +341,16 @@ BEGIN;
     select create_distributed_table('test_create_seq_table' ,'a');
 ROLLBACK;
 
+-- trigger switch-over when using single connection per worker
+BEGIN;
+SET citus.next_shard_id TO 16900;
+SET LOCAL citus.shard_count TO 4;
+SET LOCAL citus.multi_shard_modify_mode TO 'sequential';
+CREATE UNLOGGED TABLE trigger_switchover(a int, b int, c int, d int, e int, f int, g int, h int);
+INSERT INTO trigger_switchover
+  SELECT s AS a, s AS b, s AS c, s AS d, s AS e, s AS f, s AS g, s AS h FROM generate_series(1,250000) s;
+SELECT create_distributed_table('trigger_switchover','a');
+ABORT;
+
 SET search_path TO 'public';
 DROP SCHEMA test_seq_ddl CASCADE;