Error out on foreign keys with reference tables

We have one replication of reference table for each node. Therefore all problems with
replication factor > 1 also applies to reference table. As a solution we will not allow
foreign keys on reference tables. It is not possible to define foreign key from, to or
between reference tables.

src/backend/distributed/commands/create_distributed_table.c

@@ -408,12 +408,14 @@ ErrorIfNotSupportedConstraint(Relation relation, char distributionMethod,
 	List *indexOidList = NULL;
 	ListCell *indexOidCell = NULL;
 
+	/* we first perform check for foreign constraints */
+	ErrorIfNotSupportedForeignConstraint(relation, distributionMethod, distributionColumn,
+										 colocationId);
+
 	/*
 	 * Citus supports any kind of uniqueness constraints for reference tables
 	 * given that they only consist of a single shard and we can simply rely on
 	 * Postgres.
-	 * TODO: Here we should be erroring out if there exists any foreign keys
-	 * from/to a reference table.
 	 */
 	if (distributionMethod == DISTRIBUTE_BY_NONE)
 	{
@@ -499,10 +501,6 @@ ErrorIfNotSupportedConstraint(Relation relation, char distributionMethod,
 
 		index_close(indexDesc, NoLock);
 	}
-
-	/* we also perform check for foreign constraints */
-	ErrorIfNotSupportedForeignConstraint(relation, distributionMethod, distributionColumn,
-										 colocationId);
 }
 
 
@@ -560,6 +558,19 @@ ErrorIfNotSupportedForeignConstraint(Relation relation, char distributionMethod,
 			continue;
 		}
 
+		referencedTableId = constraintForm->confrelid;
+
+		/* we do not support foreign keys for reference tables */
+		if (distributionMethod == DISTRIBUTE_BY_NONE ||
+			(relation->rd_id != referencedTableId &&
+			 PartitionMethod(referencedTableId) == DISTRIBUTE_BY_NONE))
+		{
+			ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+							errmsg("cannot create foreign key constraint"),
+							errdetail("Foreign key constraints are not allowed from or "
+									  "to reference tables.")));
+		}
+
 		/*
 		 * ON DELETE SET NULL and ON DELETE SET DEFAULT is not supported. Because we do
 		 * not want to set partition column to NULL or default value.
@@ -589,8 +600,6 @@ ErrorIfNotSupportedForeignConstraint(Relation relation, char distributionMethod,
 									  " supported in ON UPDATE operation.")));
 		}
 
-		referencedTableId = constraintForm->confrelid;
-
 		/*
 		 * Some checks are not meaningful if foreign key references the table itself.
 		 * Therefore we will skip those checks.

src/backend/distributed/executor/multi_utility.c

@@ -1494,6 +1494,23 @@ ErrorIfUnsupportedAlterTableStmt(AlterTableStmt *alterTableStatement)
 									errhint("You can issue each subcommand separately")));
 				}
 
+				referencingTableId = RangeVarGetRelid(alterTableStatement->relation,
+													  lockmode,
+													  alterTableStatement->missing_ok);
+				referencedTableId = RangeVarGetRelid(constraint->pktable, lockmode,
+													 alterTableStatement->missing_ok);
+
+				/* we do not support foreign keys for reference tables */
+				if (PartitionMethod(referencingTableId) == DISTRIBUTE_BY_NONE ||
+					PartitionMethod(referencedTableId) == DISTRIBUTE_BY_NONE)
+				{
+					ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+									errmsg("cannot create foreign key constraint"),
+									errdetail(
+										"Foreign key constraints are not allowed from or "
+										"to reference tables.")));
+				}
+
 				/*
 				 * ON DELETE SET NULL and ON DELETE SET DEFAULT is not supported. Because
 				 * we do not want to set partition column to NULL or default value.
@@ -1538,12 +1555,6 @@ ErrorIfUnsupportedAlterTableStmt(AlterTableStmt *alterTableStatement)
 				}
 
 				/* to enforce foreign constraints, tables must be co-located */
-				referencingTableId = RangeVarGetRelid(alterTableStatement->relation,
-													  lockmode,
-													  alterTableStatement->missing_ok);
-				referencedTableId = RangeVarGetRelid(constraint->pktable, lockmode,
-													 alterTableStatement->missing_ok);
-
 				if (!TablesColocated(referencingTableId, referencedTableId))
 				{
 					ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),

src/test/regress/expected/multi_foreign_key.out

@@ -773,3 +773,64 @@ SELECT * FROM self_referencing_table2;
 
 -- we no longer need those tables
 DROP TABLE self_referencing_table2;
+-- test reference tables
+-- test foreign key creation on CREATE TABLE from reference table
+CREATE TABLE referenced_by_reference_table(id int PRIMARY KEY, other_column int);
+SELECT create_distributed_table('referenced_by_reference_table', 'id');
+ create_distributed_table 
+--------------------------
+ 
+(1 row)
+
+CREATE TABLE reference_table(id int, referencing_column int REFERENCES referenced_by_reference_table(id));
+SELECT create_reference_table('reference_table');
+ERROR:  cannot create foreign key constraint
+DETAIL:  Foreign key constraints are not allowed from or to reference tables.
+-- test foreign key creation on CREATE TABLE to reference table
+DROP TABLE reference_table;
+CREATE TABLE reference_table(id int PRIMARY KEY, referencing_column int);
+SELECT create_reference_table('reference_table');
+ create_reference_table 
+------------------------
+ 
+(1 row)
+
+CREATE TABLE references_to_reference_table(id int, referencing_column int REFERENCES reference_table(id));
+SELECT create_distributed_table('references_to_reference_table', 'referencing_column');
+ERROR:  cannot create foreign key constraint
+DETAIL:  Foreign key constraints are not allowed from or to reference tables.
+-- test foreign key creation on CREATE TABLE from + to reference table
+CREATE TABLE reference_table2(id int, referencing_column int REFERENCES reference_table(id));
+SELECT create_reference_table('reference_table2');
+ERROR:  cannot create foreign key constraint
+DETAIL:  Foreign key constraints are not allowed from or to reference tables.
+-- test foreign key creation on ALTER TABLE from reference table
+ALTER TABLE reference_table ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES referenced_by_reference_table(id);
+ERROR:  cannot create foreign key constraint
+DETAIL:  Foreign key constraints are not allowed from or to reference tables.
+-- test foreign key creation on ALTER TABLE to reference table
+DROP TABLE references_to_reference_table;
+CREATE TABLE references_to_reference_table(id int, referencing_column int);
+SELECT create_distributed_table('references_to_reference_table', 'referencing_column');
+ create_distributed_table 
+--------------------------
+ 
+(1 row)
+
+ALTER TABLE references_to_reference_table ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES reference_table(id);
+ERROR:  cannot create foreign key constraint
+DETAIL:  Foreign key constraints are not allowed from or to reference tables.
+-- test foreign key creation on ALTER TABLE from + to reference table
+DROP TABLE reference_table2;
+CREATE TABLE reference_table2(id int, referencing_column int);
+SELECT create_reference_table('reference_table2');
+ create_reference_table 
+------------------------
+ 
+(1 row)
+
+ALTER TABLE reference_table2 ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES reference_table(id);
+ERROR:  cannot create foreign key constraint
+DETAIL:  Foreign key constraints are not allowed from or to reference tables.
+-- we no longer need those tables
+DROP TABLE referenced_by_reference_table, references_to_reference_table, reference_table, reference_table2;

src/test/regress/sql/multi_foreign_key.sql

@@ -449,3 +449,42 @@ SELECT * FROM self_referencing_table2;
 
 -- we no longer need those tables
 DROP TABLE self_referencing_table2;
+
+
+-- test reference tables
+-- test foreign key creation on CREATE TABLE from reference table
+CREATE TABLE referenced_by_reference_table(id int PRIMARY KEY, other_column int);
+SELECT create_distributed_table('referenced_by_reference_table', 'id');
+
+CREATE TABLE reference_table(id int, referencing_column int REFERENCES referenced_by_reference_table(id));
+SELECT create_reference_table('reference_table');
+
+-- test foreign key creation on CREATE TABLE to reference table
+DROP TABLE reference_table;
+CREATE TABLE reference_table(id int PRIMARY KEY, referencing_column int);
+SELECT create_reference_table('reference_table');
+
+CREATE TABLE references_to_reference_table(id int, referencing_column int REFERENCES reference_table(id));
+SELECT create_distributed_table('references_to_reference_table', 'referencing_column');
+
+-- test foreign key creation on CREATE TABLE from + to reference table
+CREATE TABLE reference_table2(id int, referencing_column int REFERENCES reference_table(id));
+SELECT create_reference_table('reference_table2');
+
+-- test foreign key creation on ALTER TABLE from reference table
+ALTER TABLE reference_table ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES referenced_by_reference_table(id);
+
+-- test foreign key creation on ALTER TABLE to reference table
+DROP TABLE references_to_reference_table;
+CREATE TABLE references_to_reference_table(id int, referencing_column int);
+SELECT create_distributed_table('references_to_reference_table', 'referencing_column');
+ALTER TABLE references_to_reference_table ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES reference_table(id);
+
+-- test foreign key creation on ALTER TABLE from + to reference table
+DROP TABLE reference_table2;
+CREATE TABLE reference_table2(id int, referencing_column int);
+SELECT create_reference_table('reference_table2');
+ALTER TABLE reference_table2 ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES reference_table(id);
+
+-- we no longer need those tables
+DROP TABLE referenced_by_reference_table, references_to_reference_table, reference_table, reference_table2;