Address Onder's comments

src/backend/distributed/commands/create_distributed_table.c

@@ -408,7 +408,13 @@ ErrorIfNotSupportedConstraint(Relation relation, char distributionMethod,
 	List *indexOidList = NULL;
 	ListCell *indexOidCell = NULL;
 
-	/* we first perform check for foreign constraints */
+	/*
+	 * We first perform check for foreign constraints. It is important to do this check
+	 * before next check, because other types of constraints are allowed on reference
+	 * tables and we return early for those constraints thanks to next check. Therefore,
+	 * for reference tables, we first check for foreing constraints and if they are OK,
+	 * we do not error out for other types of constraints.
+	 */
 	ErrorIfNotSupportedForeignConstraint(relation, distributionMethod, distributionColumn,
 										 colocationId);
 
@@ -540,6 +546,7 @@ ErrorIfNotSupportedForeignConstraint(Relation relation, char distributionMethod,
 	bool isNull = false;
 	int attrIdx = 0;
 	bool foreignConstraintOnPartitionColumn = false;
+	bool selfReferencingTable = false;
 
 	pgConstraint = heap_open(ConstraintRelationId, AccessShareLock);
 	ScanKeyInit(&scanKey[0], Anum_pg_constraint_conrelid, BTEqualStrategyNumber, F_OIDEQ,
@@ -559,10 +566,18 @@ ErrorIfNotSupportedForeignConstraint(Relation relation, char distributionMethod,
 		}
 
 		referencedTableId = constraintForm->confrelid;
+		selfReferencingTable = relation->rd_id == referencedTableId;
 
-		/* we do not support foreign keys for reference tables */
+		/*
+		 * We do not support foreign keys for reference tables. Here we skip the second
+		 * part of check if the table is a self referencing table because;
+		 * - PartitionMethod only works for distributed tables and this table is not
+		 * distributed yet.
+		 * - Since referencing and referenced tables are same, it is OK to not checking
+		 * distribution method twice.
+		 */
 		if (distributionMethod == DISTRIBUTE_BY_NONE ||
-			(relation->rd_id != referencedTableId &&
+			(!selfReferencingTable &&
 			 PartitionMethod(referencedTableId) == DISTRIBUTE_BY_NONE))
 		{
 			ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
@@ -604,7 +619,7 @@ ErrorIfNotSupportedForeignConstraint(Relation relation, char distributionMethod,
 		 * Some checks are not meaningful if foreign key references the table itself.
 		 * Therefore we will skip those checks.
 		 */
-		if (referencedTableId != relation->rd_id)
+		if (!selfReferencingTable)
 		{
 			if (!IsDistributedTable(referencedTableId))
 			{

src/test/regress/expected/multi_foreign_key.out

@@ -800,11 +800,40 @@ SELECT create_distributed_table('references_to_reference_table', 'referencing_co
 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));
+CREATE TABLE reference_table_second(id int, referencing_column int REFERENCES reference_table(id));
-SELECT create_reference_table('reference_table2');
+SELECT create_reference_table('reference_table_second');
+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 reference table to local table
+CREATE TABLE referenced_local_table(id int PRIMARY KEY, other_column int);
+DROP TABLE reference_table CASCADE;
+NOTICE:  drop cascades to 2 other objects
+DETAIL:  drop cascades to constraint references_to_reference_table_referencing_column_fkey on table references_to_reference_table
+drop cascades to constraint reference_table_second_referencing_column_fkey on table reference_table_second
+CREATE TABLE reference_table(id int, referencing_column int REFERENCES referenced_local_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 on self referencing reference table
+CREATE TABLE self_referencing_reference_table(
+    id int,
+    other_column int,
+    other_column_ref int,
+    PRIMARY KEY(id, other_column),
+    FOREIGN KEY(id, other_column_ref) REFERENCES self_referencing_reference_table(id, other_column)
+);
+SELECT create_reference_table('self_referencing_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 ALTER TABLE from 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)
+
 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.
@@ -821,16 +850,45 @@ ALTER TABLE references_to_reference_table ADD CONSTRAINT fk FOREIGN KEY(referenc
 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;
+DROP TABLE reference_table_second;
-CREATE TABLE reference_table2(id int, referencing_column int);
+CREATE TABLE reference_table_second(id int, referencing_column int);
-SELECT create_reference_table('reference_table2');
+SELECT create_reference_table('reference_table_second');
  create_reference_table 
 ------------------------
  
 (1 row)
 
-ALTER TABLE reference_table2 ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES reference_table(id);
+ALTER TABLE reference_table_second 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 reference table to local 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)
+
+ALTER TABLE reference_table ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES referenced_local_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 on self referencing reference table
+DROP TABLE self_referencing_reference_table;
+CREATE TABLE self_referencing_reference_table(
+    id int,
+    other_column int,
+    other_column_ref int,
+    PRIMARY KEY(id, other_column)
+);
+SELECT create_reference_table('self_referencing_reference_table');
+ create_reference_table 
+------------------------
+ 
+(1 row)
+
+ALTER TABLE self_referencing_reference_table ADD CONSTRAINT fk FOREIGN KEY(id, other_column_ref) REFERENCES self_referencing_reference_table(id, other_column);
 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;
+DROP TABLE referenced_by_reference_table, references_to_reference_table, reference_table, reference_table_second, referenced_local_table, self_referencing_reference_table;

src/test/regress/sql/multi_foreign_key.sql

@@ -468,10 +468,29 @@ CREATE TABLE references_to_reference_table(id int, referencing_column int REFERE
 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));
+CREATE TABLE reference_table_second(id int, referencing_column int REFERENCES reference_table(id));
-SELECT create_reference_table('reference_table2');
+SELECT create_reference_table('reference_table_second');
+
+-- test foreign key creation on CREATE TABLE from reference table to local table
+CREATE TABLE referenced_local_table(id int PRIMARY KEY, other_column int);
+DROP TABLE reference_table CASCADE;
+CREATE TABLE reference_table(id int, referencing_column int REFERENCES referenced_local_table(id));
+SELECT create_reference_table('reference_table');
+
+-- test foreign key creation on CREATE TABLE on self referencing reference table
+CREATE TABLE self_referencing_reference_table(
+    id int,
+    other_column int,
+    other_column_ref int,
+    PRIMARY KEY(id, other_column),
+    FOREIGN KEY(id, other_column_ref) REFERENCES self_referencing_reference_table(id, other_column)
+);
+SELECT create_reference_table('self_referencing_reference_table');
 
 -- test foreign key creation on ALTER TABLE from reference table
+DROP TABLE reference_table;
+CREATE TABLE reference_table(id int PRIMARY KEY, referencing_column int);
+SELECT create_reference_table('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
@@ -481,10 +500,27 @@ SELECT create_distributed_table('references_to_reference_table', 'referencing_co
 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;
+DROP TABLE reference_table_second;
-CREATE TABLE reference_table2(id int, referencing_column int);
+CREATE TABLE reference_table_second(id int, referencing_column int);
-SELECT create_reference_table('reference_table2');
+SELECT create_reference_table('reference_table_second');
-ALTER TABLE reference_table2 ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES reference_table(id);
+ALTER TABLE reference_table_second ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES reference_table(id);
+
+-- test foreign key creation on ALTER TABLE from reference table to local table
+DROP TABLE reference_table;
+CREATE TABLE reference_table(id int PRIMARY KEY, referencing_column int);
+SELECT create_reference_table('reference_table');
+ALTER TABLE reference_table ADD CONSTRAINT fk FOREIGN KEY(referencing_column) REFERENCES referenced_local_table(id);
+
+-- test foreign key creation on ALTER TABLE on self referencing reference table
+DROP TABLE self_referencing_reference_table;
+CREATE TABLE self_referencing_reference_table(
+    id int,
+    other_column int,
+    other_column_ref int,
+    PRIMARY KEY(id, other_column)
+);
+SELECT create_reference_table('self_referencing_reference_table');
+ALTER TABLE self_referencing_reference_table ADD CONSTRAINT fk FOREIGN KEY(id, other_column_ref) REFERENCES self_referencing_reference_table(id, other_column);
 
 -- we no longer need those tables
-DROP TABLE referenced_by_reference_table, references_to_reference_table, reference_table, reference_table2;
+DROP TABLE referenced_by_reference_table, references_to_reference_table, reference_table, reference_table_second, referenced_local_table, self_referencing_reference_table;