Add enable_ddl_propagation flag to control automatic ddl propagation

src/backend/distributed/executor/multi_utility.c

@@ -40,6 +40,8 @@
 #include "utils/syscache.h"
 
 
+bool EnableDDLPropagation = true; /* ddl propagation is enabled */
+
 /*
  * This struct defines the state for the callback for drop statements.
  * It is copied as it is from commands/tablecmds.c in Postgres source.
@@ -145,6 +147,9 @@ multi_ProcessUtility(Node *parsetree,
 		}
 	}
 
+	/* ddl commands are propagated to workers only if EnableDDLPropagation is set */
+	if (EnableDDLPropagation)
+	{
 		if (IsA(parsetree, IndexStmt))
 		{
 			parsetree = ProcessIndexStmt((IndexStmt *) parsetree, queryString);
@@ -180,6 +185,7 @@ multi_ProcessUtility(Node *parsetree,
 				ErrorIfDistributedRenameStmt(renameStmt);
 			}
 		}
+	}
 
 	/*
 	 * Inform the user about potential caveats.

src/backend/distributed/shared_library_init.c

@@ -308,6 +308,16 @@ RegisterCitusConfigVariables(void)
 		0,
 		NULL, NULL, NULL);
 
+	DefineCustomBoolVariable(
+		"citus.enable_ddl_propagation",
+		gettext_noop("Enables propagating DDL statements to worker shards"),
+		NULL,
+		&EnableDDLPropagation,
+		true,
+		PGC_USERSET,
+		0,
+		NULL, NULL, NULL);
+
 	DefineCustomIntVariable(
 		"citus.shard_replication_factor",
 		gettext_noop("Sets the replication factor for shards."),

src/include/distributed/multi_utility.h

@@ -12,6 +12,7 @@
 
 #include "tcop/utility.h"
 
+extern bool EnableDDLPropagation;
 
 extern void multi_ProcessUtility(Node *parsetree, const char *queryString,
 								 ProcessUtilityContext context, ParamListInfo params,

src/test/regress/input/multi_alter_table_statements.source

@@ -162,7 +162,56 @@ FROM
 ORDER BY attnum;
 \c - - - :master_port
 
+-- verify that we don't intercept DDL commands if propagation is turned off
+SET citus.enable_ddl_propagation to false;
+
+-- table rename statement can be performed now
+ALTER TABLE lineitem_alter RENAME TO lineitem_renamed;
+-- verify rename is performed
+SELECT relname FROM pg_class WHERE relname = 'lineitem_alter' or relname = 'lineitem_renamed';
+
+-- revert it to original name
+ALTER TABLE lineitem_renamed RENAME TO lineitem_alter;
+
+-- this column is added to master table and not workers
+ALTER TABLE lineitem_alter ADD COLUMN column_only_added_to_master int;
+
+-- verify newly added column is not present in a worker shard
+\c - - - :worker_1_port
+SELECT column_only_added_to_master FROM lineitem_alter_103000 LIMIT 0;
+\c - - - :master_port
+
+-- ddl propagation flag is reset to default, disable it again
+SET citus.enable_ddl_propagation to false;
+
+-- following query succeeds since it accesses an previously existing column
+SELECT l_orderkey FROM lineitem_alter LIMIT 0;
+
+-- make master and workers have the same schema again
+ALTER TABLE lineitem_alter DROP COLUMN column_only_added_to_master;
+-- now this should succeed
+SELECT * FROM lineitem_alter LIMIT 0;
+
+-- previously unsupported statements are accepted by postgresql now
+ALTER TABLE lineitem_alter ALTER COLUMN l_orderkey SET STATISTICS 100;
+ALTER TABLE lineitem_alter DROP CONSTRAINT IF EXISTS non_existent_contraint;
+ALTER TABLE lineitem_alter SET WITHOUT OIDS;
+
+-- even distribution column can be dropped however postgresql prevents this.
+ALTER TABLE lineitem_alter DROP COLUMN l_orderkey;
+
+-- Even unique indexes on l_partkey (non-partition column) are allowed.
+-- Citus would have prevented that.
+CREATE UNIQUE INDEX unique_lineitem_partkey on lineitem_alter(l_partkey);
+SELECT  indexname, tablename FROM pg_indexes WHERE tablename = 'lineitem_alter';
+
+-- verify index is not created on worker
+\c - - - :worker_1_port
+SELECT  indexname, tablename FROM pg_indexes WHERE tablename like 'lineitem_alter_%';
+\c - - - :master_port
+
 -- Cleanup the table and its shards
+SET citus.enable_ddl_propagation to true;
 SELECT master_apply_delete_command('DELETE FROM lineitem_alter');
 DROP TABLE lineitem_alter;
 -- check that nothing's left over on workers

src/test/regress/output/multi_alter_table_statements.source

@@ -451,8 +451,74 @@ ORDER BY attnum;
  ........pg.dropped.23........ | -
 (29 rows)
 
+\c - - - :master_port
+-- verify that we don't intercept DDL commands if propagation is turned off
+SET citus.enable_ddl_propagation to false;
+-- table rename statement can be performed now
+ALTER TABLE lineitem_alter RENAME TO lineitem_renamed;
+-- verify rename is performed
+SELECT relname FROM pg_class WHERE relname = 'lineitem_alter' or relname = 'lineitem_renamed';
+     relname      
+------------------
+ lineitem_renamed
+(1 row)
+
+-- revert it to original name
+ALTER TABLE lineitem_renamed RENAME TO lineitem_alter;
+-- this column is added to master table and not workers
+ALTER TABLE lineitem_alter ADD COLUMN column_only_added_to_master int;
+-- verify newly added column is not present in a worker shard
+\c - - - :worker_1_port
+SELECT column_only_added_to_master FROM lineitem_alter_103000 LIMIT 0;
+ERROR:  column "column_only_added_to_master" does not exist
+LINE 1: SELECT column_only_added_to_master FROM lineitem_alter_10300...
+               ^
+\c - - - :master_port
+-- ddl propagation flag is reset to default, disable it again
+SET citus.enable_ddl_propagation to false;
+-- following query succeeds since it accesses an previously existing column
+SELECT l_orderkey FROM lineitem_alter LIMIT 0;
+ l_orderkey 
+------------
+(0 rows)
+
+-- make master and workers have the same schema again
+ALTER TABLE lineitem_alter DROP COLUMN column_only_added_to_master;
+-- now this should succeed
+SELECT * FROM lineitem_alter LIMIT 0;
+ l_orderkey | l_partkey | l_suppkey | l_linenumber | l_quantity | l_extendedprice | l_discount | l_tax | l_returnflag | l_linestatus | l_shipdate | l_commitdate | l_receiptdate | l_shipinstruct | l_shipmode | l_comment | null_column 
+------------+-----------+-----------+--------------+------------+-----------------+------------+-------+--------------+--------------+------------+--------------+---------------+----------------+------------+-----------+-------------
+(0 rows)
+
+-- previously unsupported statements are accepted by postgresql now
+ALTER TABLE lineitem_alter ALTER COLUMN l_orderkey SET STATISTICS 100;
+ALTER TABLE lineitem_alter DROP CONSTRAINT IF EXISTS non_existent_contraint;
+NOTICE:  constraint "non_existent_contraint" of relation "lineitem_alter" does not exist, skipping
+ALTER TABLE lineitem_alter SET WITHOUT OIDS;
+-- even distribution column can be dropped however postgresql prevents this.
+ALTER TABLE lineitem_alter DROP COLUMN l_orderkey;
+ERROR:  cannot drop table lineitem_alter column l_orderkey because other objects depend on it
+DETAIL:  table lineitem_alter depends on table lineitem_alter column l_orderkey
+HINT:  Use DROP ... CASCADE to drop the dependent objects too.
+-- Even unique indexes on l_partkey (non-partition column) are allowed.
+-- Citus would have prevented that.
+CREATE UNIQUE INDEX unique_lineitem_partkey on lineitem_alter(l_partkey);
+SELECT  indexname, tablename FROM pg_indexes WHERE tablename = 'lineitem_alter';
+        indexname        |   tablename    
+-------------------------+----------------
+ unique_lineitem_partkey | lineitem_alter
+(1 row)
+
+-- verify index is not created on worker
+\c - - - :worker_1_port
+SELECT  indexname, tablename FROM pg_indexes WHERE tablename like 'lineitem_alter_%';
+ indexname | tablename 
+-----------+-----------
+(0 rows)
+
 \c - - - :master_port
 -- Cleanup the table and its shards
+SET citus.enable_ddl_propagation to true;
 SELECT master_apply_delete_command('DELETE FROM lineitem_alter');
  master_apply_delete_command 
 -----------------------------