Generate a WARNING when the column types in a foreign key constraint are

incompatible enough to prevent indexscanning the referenced table.  Also,
improve the error message that pops out when we can't implement the FK at
all for lack of a usable equality operator.  Fabien Coelho, with some review
by Tom Lane.

src/backend/commands/tablecmds.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.99 2004/02/15 21:01:39 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.100 2004/03/13 22:09:13 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -89,10 +89,12 @@ static void AlterTableAddForeignKeyConstraint(Relation rel,
 static int transformColumnNameList(Oid relId, List *colList,
 						int16 *attnums, Oid *atttypids);
 static int transformFkeyGetPrimaryKey(Relation pkrel, Oid *indexOid,
-						   List **attnamelist,
-						   int16 *attnums, Oid *atttypids);
+									  List **attnamelist,
+									  int16 *attnums, Oid *atttypids,
+									  Oid *opclasses);
 static Oid transformFkeyCheckAttrs(Relation pkrel,
-						int numattrs, int16 *attnums);
+								   int numattrs, int16 *attnums,
+								   Oid *opclasses);
 static void validateForeignKeyConstraint(FkConstraint *fkconstraint,
 							 Relation rel, Relation pkrel);
 static void createForeignKeyTriggers(Relation rel, FkConstraint *fkconstraint,
@@ -3016,6 +3018,7 @@ AlterTableAddForeignKeyConstraint(Relation rel, FkConstraint *fkconstraint)
 	int16		fkattnum[INDEX_MAX_KEYS];
 	Oid			pktypoid[INDEX_MAX_KEYS];
 	Oid			fktypoid[INDEX_MAX_KEYS];
+	Oid         opclasses[INDEX_MAX_KEYS];
 	int			i;
 	int			numfks,
 				numpks;
@@ -3088,11 +3091,11 @@ AlterTableAddForeignKeyConstraint(Relation rel, FkConstraint *fkconstraint)
 	 * Look up the referencing attributes to make sure they exist, and
 	 * record their attnums and type OIDs.
 	 */
-	for (i = 0; i < INDEX_MAX_KEYS; i++)
-	{
-		pkattnum[i] = fkattnum[i] = 0;
-		pktypoid[i] = fktypoid[i] = InvalidOid;
-	}
+	MemSet(pkattnum, 0, sizeof(pkattnum));
+	MemSet(fkattnum, 0, sizeof(fkattnum));
+	MemSet(pktypoid, 0, sizeof(pktypoid));
+	MemSet(fktypoid, 0, sizeof(fktypoid));
+	MemSet(opclasses, 0, sizeof(opclasses));
 
 	numfks = transformColumnNameList(RelationGetRelid(rel),
 									 fkconstraint->fk_attrs,
@@ -3102,13 +3105,15 @@ AlterTableAddForeignKeyConstraint(Relation rel, FkConstraint *fkconstraint)
 	 * If the attribute list for the referenced table was omitted, lookup
 	 * the definition of the primary key and use it.  Otherwise, validate
 	 * the supplied attribute list.  In either case, discover the index
-	 * OID and the attnums and type OIDs of the attributes.
+	 * OID and index opclasses, and the attnums and type OIDs of the
+	 * attributes.
 	 */
 	if (fkconstraint->pk_attrs == NIL)
 	{
 		numpks = transformFkeyGetPrimaryKey(pkrel, &indexOid,
 											&fkconstraint->pk_attrs,
-											pkattnum, pktypoid);
+											pkattnum, pktypoid,
+											opclasses);
 	}
 	else
 	{
@@ -3116,7 +3121,8 @@ AlterTableAddForeignKeyConstraint(Relation rel, FkConstraint *fkconstraint)
 										 fkconstraint->pk_attrs,
 										 pkattnum, pktypoid);
 		/* Look for an index matching the column list */
-		indexOid = transformFkeyCheckAttrs(pkrel, numpks, pkattnum);
+		indexOid = transformFkeyCheckAttrs(pkrel, numpks, pkattnum,
+										   opclasses);
 	}
 
 	/* Be sure referencing and referenced column types are comparable */
@@ -3128,14 +3134,48 @@ AlterTableAddForeignKeyConstraint(Relation rel, FkConstraint *fkconstraint)
 	for (i = 0; i < numpks; i++)
 	{
 		/*
-		 * fktypoid[i] is the foreign key table's i'th element's type
-		 * pktypoid[i] is the primary key table's i'th element's type
+		 * pktypoid[i] is the primary key table's i'th key's type
+		 * fktypoid[i] is the foreign key table's i'th key's type
 		 *
-		 * We let oper() do our work for us, including ereport(ERROR) if the
-		 * types don't compare with =
+		 * Note that we look for an operator with the PK type on the left;
+		 * when the types are different this is critical because the PK index
+		 * will need operators with the indexkey on the left.  (Ordinarily
+		 * both commutator operators will exist if either does, but we won't
+		 * get the right answer from the test below on opclass membership
+		 * unless we select the proper operator.)
 		 */
 		Operator	o = oper(makeList1(makeString("=")),
-							 fktypoid[i], pktypoid[i], false);
+							 pktypoid[i], fktypoid[i], true);
+
+		if (o == NULL)
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_FUNCTION),
+					 errmsg("foreign key constraint \"%s\" "
+							"cannot be implemented",
+							fkconstraint->constr_name),
+					 errdetail("Key columns \"%s\" and \"%s\" "
+							   "are of incompatible types: %s and %s.",
+							   strVal(nth(i, fkconstraint->fk_attrs)),
+							   strVal(nth(i, fkconstraint->pk_attrs)),
+							   format_type_be(fktypoid[i]),
+							   format_type_be(pktypoid[i]))));
+
+		/*
+		 * Check that the found operator is compatible with the PK index,
+		 * and generate a warning if not, since otherwise costly seqscans
+		 * will be incurred to check FK validity.
+		 */
+		if (!op_in_opclass(oprid(o), opclasses[i]))
+			ereport(WARNING, 
+					(errmsg("foreign key constraint \"%s\" "
+							"will require costly sequential scans",
+							fkconstraint->constr_name),
+					 errdetail("Key columns \"%s\" and \"%s\" "
+							   "are of different types: %s and %s.",
+							   strVal(nth(i, fkconstraint->fk_attrs)),
+							   strVal(nth(i, fkconstraint->pk_attrs)),
+							   format_type_be(fktypoid[i]),
+							   format_type_be(pktypoid[i]))));
 
 		ReleaseSysCache(o);
 	}
@@ -3225,13 +3265,19 @@ transformColumnNameList(Oid relId, List *colList,
  * transformFkeyGetPrimaryKey -
  *
  *	Look up the names, attnums, and types of the primary key attributes
- *	for the pkrel.	Used when the column list in the REFERENCES specification
- *	is omitted.
+ *	for the pkrel.  Also return the index OID and index opclasses of the
+ *	index supporting the primary key.
+ *
+ *	All parameters except pkrel are output parameters.  Also, the function
+ *	return value is the number of attributes in the primary key.
+ *
+ *	Used when the column list in the REFERENCES specification is omitted.
  */
 static int
 transformFkeyGetPrimaryKey(Relation pkrel, Oid *indexOid,
 						   List **attnamelist,
-						   int16 *attnums, Oid *atttypids)
+						   int16 *attnums, Oid *atttypids,
+						   Oid *opclasses)
 {
 	List	   *indexoidlist,
 			   *indexoidscan;
@@ -3287,6 +3333,7 @@ transformFkeyGetPrimaryKey(Relation pkrel, Oid *indexOid,
 
 		attnums[i] = pkattno;
 		atttypids[i] = attnumTypeId(pkrel, pkattno);
+		opclasses[i] = indexStruct->indclass[i];
 		*attnamelist = lappend(*attnamelist,
 		   makeString(pstrdup(NameStr(*attnumAttName(pkrel, pkattno)))));
 	}
@@ -3301,11 +3348,13 @@ transformFkeyGetPrimaryKey(Relation pkrel, Oid *indexOid,
  *
  *	Make sure that the attributes of a referenced table belong to a unique
  *	(or primary key) constraint.  Return the OID of the index supporting
- *	the constraint.
+ *	the constraint, as well as the opclasses associated with the index
+ *	columns.
  */
 static Oid
 transformFkeyCheckAttrs(Relation pkrel,
-						int numattrs, int16 *attnums)
+						int numattrs, int16 *attnums,
+						Oid *opclasses)			/* output parameter */
 {
 	Oid			indexoid = InvalidOid;
 	bool		found = false;
@@ -3370,6 +3419,7 @@ transformFkeyCheckAttrs(Relation pkrel,
 					{
 						if (attnums[j] == indexStruct->indkey[i])
 						{
+							opclasses[j] = indexStruct->indclass[i];
 							found = true;
 							break;
 						}

src/test/regress/expected/alter_table.out

@@ -194,20 +194,24 @@ NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "pktable_pkey" fo
 CREATE TEMP TABLE FKTABLE (ftest1 inet);
 -- This next should fail, because inet=int does not exist
 ALTER TABLE FKTABLE ADD FOREIGN KEY(ftest1) references pktable;
-ERROR:  operator does not exist: inet = integer
-HINT:  No operator matches the given name and argument type(s). You may need to add explicit type casts.
+ERROR:  foreign key constraint "$1" cannot be implemented
+DETAIL:  Key columns "ftest1" and "ptest1" are of incompatible types: inet and integer.
 -- This should also fail for the same reason, but here we
 -- give the column name
 ALTER TABLE FKTABLE ADD FOREIGN KEY(ftest1) references pktable(ptest1);
-ERROR:  operator does not exist: inet = integer
-HINT:  No operator matches the given name and argument type(s). You may need to add explicit type casts.
+ERROR:  foreign key constraint "$1" cannot be implemented
+DETAIL:  Key columns "ftest1" and "ptest1" are of incompatible types: inet and integer.
 -- This should succeed, even though they are different types
 -- because varchar=int does exist
 DROP TABLE FKTABLE;
 CREATE TEMP TABLE FKTABLE (ftest1 varchar);
 ALTER TABLE FKTABLE ADD FOREIGN KEY(ftest1) references pktable;
+WARNING:  foreign key constraint "$1" will require costly sequential scans
+DETAIL:  Key columns "ftest1" and "ptest1" are of different types: character varying and integer.
 -- As should this
 ALTER TABLE FKTABLE ADD FOREIGN KEY(ftest1) references pktable(ptest1);
+WARNING:  foreign key constraint "$2" will require costly sequential scans
+DETAIL:  Key columns "ftest1" and "ptest1" are of different types: character varying and integer.
 DROP TABLE pktable cascade;
 NOTICE:  drop cascades to constraint $2 on table fktable
 NOTICE:  drop cascades to constraint $1 on table fktable
@@ -218,27 +222,27 @@ NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "pktable_pkey" fo
 -- This should fail, because we just chose really odd types
 CREATE TEMP TABLE FKTABLE (ftest1 cidr, ftest2 timestamp);
 ALTER TABLE FKTABLE ADD FOREIGN KEY(ftest1, ftest2) references pktable;
-ERROR:  operator does not exist: cidr = integer
-HINT:  No operator matches the given name and argument type(s). You may need to add explicit type casts.
+ERROR:  foreign key constraint "$1" cannot be implemented
+DETAIL:  Key columns "ftest1" and "ptest1" are of incompatible types: cidr and integer.
 DROP TABLE FKTABLE;
 -- Again, so should this...
 CREATE TEMP TABLE FKTABLE (ftest1 cidr, ftest2 timestamp);
 ALTER TABLE FKTABLE ADD FOREIGN KEY(ftest1, ftest2)
      references pktable(ptest1, ptest2);
-ERROR:  operator does not exist: cidr = integer
-HINT:  No operator matches the given name and argument type(s). You may need to add explicit type casts.
+ERROR:  foreign key constraint "$1" cannot be implemented
+DETAIL:  Key columns "ftest1" and "ptest1" are of incompatible types: cidr and integer.
 DROP TABLE FKTABLE;
 -- This fails because we mixed up the column ordering
 CREATE TEMP TABLE FKTABLE (ftest1 int, ftest2 inet);
 ALTER TABLE FKTABLE ADD FOREIGN KEY(ftest1, ftest2)
      references pktable(ptest2, ptest1);
-ERROR:  operator does not exist: integer = inet
-HINT:  No operator matches the given name and argument type(s). You may need to add explicit type casts.
+ERROR:  foreign key constraint "$1" cannot be implemented
+DETAIL:  Key columns "ftest1" and "ptest2" are of incompatible types: integer and inet.
 -- As does this...
 ALTER TABLE FKTABLE ADD FOREIGN KEY(ftest2, ftest1)
      references pktable(ptest1, ptest2);
-ERROR:  operator does not exist: inet = integer
-HINT:  No operator matches the given name and argument type(s). You may need to add explicit type casts.
+ERROR:  foreign key constraint "$1" cannot be implemented
+DETAIL:  Key columns "ftest2" and "ptest1" are of incompatible types: inet and integer.
 -- temp tables should go away by themselves, need not drop them.
 -- test check constraint adding
 create table atacc1 ( test int );

src/test/regress/sql/foreign_key.sql

@@ -443,8 +443,8 @@ CREATE TABLE FKTABLE (ftest1 inet REFERENCES pktable);
 -- This should also fail for the same reason, but here we
 -- give the column name
 CREATE TABLE FKTABLE (ftest1 inet REFERENCES pktable(ptest1));
--- This should succeed, even though they are different types
--- because varchar=int does exist
+-- This should succeed (with a warning), even though they are different types
+-- because int=varchar does exist
 CREATE TABLE FKTABLE (ftest1 varchar REFERENCES pktable);
 DROP TABLE FKTABLE;
 -- As should this
@@ -698,3 +698,73 @@ INSERT INTO fktable VALUES (100, 200);
 
 -- error here on commit
 COMMIT;
+
+-- test notice about expensive referential integrity checks,
+-- where the index cannot be used because of type incompatibilities.
+
+CREATE TEMP TABLE pktable (
+        id1     INT4 PRIMARY KEY,
+        id2     VARCHAR(4) UNIQUE,
+        id3     REAL UNIQUE,
+        UNIQUE(id1, id2, id3)
+);
+
+CREATE TEMP TABLE fktable (
+        x1      INT4 REFERENCES pktable(id1),
+        x2      VARCHAR(4) REFERENCES pktable(id2),
+        x3      REAL REFERENCES pktable(id3),
+        x4      TEXT,
+        x5      INT2
+);
+
+-- check individual constraints with alter table.
+
+-- should generate warnings
+
+ALTER TABLE fktable ADD CONSTRAINT fk_2_3
+FOREIGN KEY (x2) REFERENCES pktable(id3);
+
+ALTER TABLE fktable ADD CONSTRAINT fk_2_1
+FOREIGN KEY (x2) REFERENCES pktable(id1);
+
+ALTER TABLE fktable ADD CONSTRAINT fk_3_1
+FOREIGN KEY (x3) REFERENCES pktable(id1);
+
+-- should NOT generate warnings
+
+-- int4 promotes to text, so this is ok
+ALTER TABLE fktable ADD CONSTRAINT fk_1_2
+FOREIGN KEY (x1) REFERENCES pktable(id2);
+
+-- int4 promotes to real
+ALTER TABLE fktable ADD CONSTRAINT fk_1_3
+FOREIGN KEY (x1) REFERENCES pktable(id3);
+
+-- text is compatible with varchar
+ALTER TABLE fktable ADD CONSTRAINT fk_4_2
+FOREIGN KEY (x4) REFERENCES pktable(id2);
+
+-- int2 is part of int4 opclass as of 7.5
+ALTER TABLE fktable ADD CONSTRAINT fk_5_1
+FOREIGN KEY (x5) REFERENCES pktable(id1);
+
+-- check multikey cases, especially out-of-order column lists
+
+-- no warnings here
+
+ALTER TABLE fktable ADD CONSTRAINT fk_123_123
+FOREIGN KEY (x1,x2,x3) REFERENCES pktable(id1,id2,id3);
+
+ALTER TABLE fktable ADD CONSTRAINT fk_213_213
+FOREIGN KEY (x2,x1,x3) REFERENCES pktable(id2,id1,id3);
+
+ALTER TABLE fktable ADD CONSTRAINT fk_253_213
+FOREIGN KEY (x2,x5,x3) REFERENCES pktable(id2,id1,id3);
+
+-- warnings here
+
+ALTER TABLE fktable ADD CONSTRAINT fk_123_231
+FOREIGN KEY (x1,x2,x3) REFERENCES pktable(id2,id3,id1);
+
+ALTER TABLE fktable ADD CONSTRAINT fk_241_132
+FOREIGN KEY (x2,x4,x1) REFERENCES pktable(id1,id3,id2);