Fully enforce uniqueness of constraint names.

It's been true for a long time that we expect names of table and domain
constraints to be unique among the constraints of that table or domain.
However, the enforcement of that has been pretty haphazard, and it missed
some corner cases such as creating a CHECK constraint and then an index
constraint of the same name (as per recent report from André Hänsel).
Also, due to the lack of an actual unique index enforcing this, duplicates
could be created through race conditions.

Moreover, the code that searches pg_constraint has been quite inconsistent
about how to handle duplicate names if one did occur: some places checked
and threw errors if there was more than one match, while others just
processed the first match they came to.

To fix, create a unique index on (conrelid, contypid, conname).  Since
either conrelid or contypid is zero, this will separately enforce
uniqueness of constraint names among constraints of any one table and any
one domain.  (If we ever implement SQL assertions, and put them into this
catalog, more thought might be needed.  But it'd be at least as reasonable
to put them into a new catalog; having overloaded this one catalog with
two kinds of constraints was a mistake already IMO.)  This index can replace
the existing non-unique index on conrelid, though we need to keep the one
on contypid for query performance reasons.

Having done that, we can simplify the logic in various places that either
coped with duplicates or neglected to, as well as potentially improve
lookup performance when searching for a constraint by name.

Also, as per our usual practice, install a preliminary check so that you
get something more friendly than a unique-index violation report in the
case complained of by André.  And teach ChooseIndexName to avoid choosing
autogenerated names that would draw such a failure.

While it's not possible to make such a change in the back branches,
it doesn't seem quite too late to put this into v11, so do so.

Discussion: https://postgr.es/m/0c1001d4428f$0942b430$1bc81c90$@webkr.de

14 files changed, 446 insertions, 350 deletions

diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
index 910d3db063f..5206d5e5162 100644
--- a/src/backend/catalog/heap.c
+++ b/src/backend/catalog/heap.c
@@ -2708,7 +2708,7 @@ MergeWithExistingConstraint(Relation rel, const char *ccname, Node *expr,
 	bool		found;
 	Relation	conDesc;
 	SysScanDesc conscan;
-	ScanKeyData skey[2];
+	ScanKeyData skey[3];
 	HeapTuple	tup;
 
 	/* Search for a pg_constraint entry with same name and relation */
@@ -2717,120 +2717,120 @@ MergeWithExistingConstraint(Relation rel, const char *ccname, Node *expr,
 	found = false;
 
 	ScanKeyInit(&skey[0],
+				Anum_pg_constraint_conrelid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(RelationGetRelid(rel)));
+	ScanKeyInit(&skey[1],
+				Anum_pg_constraint_contypid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(InvalidOid));
+	ScanKeyInit(&skey[2],
 				Anum_pg_constraint_conname,
 				BTEqualStrategyNumber, F_NAMEEQ,
 				CStringGetDatum(ccname));
 
-	ScanKeyInit(&skey[1],
-				Anum_pg_constraint_connamespace,
-				BTEqualStrategyNumber, F_OIDEQ,
-				ObjectIdGetDatum(RelationGetNamespace(rel)));
-
-	conscan = systable_beginscan(conDesc, ConstraintNameNspIndexId, true,
-								 NULL, 2, skey);
+	conscan = systable_beginscan(conDesc, ConstraintRelidTypidNameIndexId, true,
+								 NULL, 3, skey);
 
-	while (HeapTupleIsValid(tup = systable_getnext(conscan)))
+	/* There can be at most one matching row */
+	if (HeapTupleIsValid(tup = systable_getnext(conscan)))
 	{
 		Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(tup);
 
-		if (con->conrelid == RelationGetRelid(rel))
+		/* Found it.  Conflicts if not identical check constraint */
+		if (con->contype == CONSTRAINT_CHECK)
 		{
-			/* Found it.  Conflicts if not identical check constraint */
-			if (con->contype == CONSTRAINT_CHECK)
-			{
-				Datum		val;
-				bool		isnull;
-
-				val = fastgetattr(tup,
-								  Anum_pg_constraint_conbin,
-								  conDesc->rd_att, &isnull);
-				if (isnull)
-					elog(ERROR, "null conbin for rel %s",
-						 RelationGetRelationName(rel));
-				if (equal(expr, stringToNode(TextDatumGetCString(val))))
-					found = true;
-			}
+			Datum		val;
+			bool		isnull;
+
+			val = fastgetattr(tup,
+							  Anum_pg_constraint_conbin,
+							  conDesc->rd_att, &isnull);
+			if (isnull)
+				elog(ERROR, "null conbin for rel %s",
+					 RelationGetRelationName(rel));
+			if (equal(expr, stringToNode(TextDatumGetCString(val))))
+				found = true;
+		}
 
-			/*
-			 * If the existing constraint is purely inherited (no local
-			 * definition) then interpret addition of a local constraint as a
-			 * legal merge.  This allows ALTER ADD CONSTRAINT on parent and
-			 * child tables to be given in either order with same end state.
-			 * However if the relation is a partition, all inherited
-			 * constraints are always non-local, including those that were
-			 * merged.
-			 */
-			if (is_local && !con->conislocal && !rel->rd_rel->relispartition)
-				allow_merge = true;
+		/*
+		 * If the existing constraint is purely inherited (no local
+		 * definition) then interpret addition of a local constraint as a
+		 * legal merge.  This allows ALTER ADD CONSTRAINT on parent and child
+		 * tables to be given in either order with same end state.  However if
+		 * the relation is a partition, all inherited constraints are always
+		 * non-local, including those that were merged.
+		 */
+		if (is_local && !con->conislocal && !rel->rd_rel->relispartition)
+			allow_merge = true;
 
-			if (!found || !allow_merge)
-				ereport(ERROR,
-						(errcode(ERRCODE_DUPLICATE_OBJECT),
-						 errmsg("constraint \"%s\" for relation \"%s\" already exists",
-								ccname, RelationGetRelationName(rel))));
+		if (!found || !allow_merge)
+			ereport(ERROR,
+					(errcode(ERRCODE_DUPLICATE_OBJECT),
+					 errmsg("constraint \"%s\" for relation \"%s\" already exists",
+							ccname, RelationGetRelationName(rel))));
 
-			/* If the child constraint is "no inherit" then cannot merge */
-			if (con->connoinherit)
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
-						 errmsg("constraint \"%s\" conflicts with non-inherited constraint on relation \"%s\"",
-								ccname, RelationGetRelationName(rel))));
+		/* If the child constraint is "no inherit" then cannot merge */
+		if (con->connoinherit)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+					 errmsg("constraint \"%s\" conflicts with non-inherited constraint on relation \"%s\"",
+							ccname, RelationGetRelationName(rel))));
 
-			/*
-			 * Must not change an existing inherited constraint to "no
-			 * inherit" status.  That's because inherited constraints should
-			 * be able to propagate to lower-level children.
-			 */
-			if (con->coninhcount > 0 && is_no_inherit)
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
-						 errmsg("constraint \"%s\" conflicts with inherited constraint on relation \"%s\"",
-								ccname, RelationGetRelationName(rel))));
+		/*
+		 * Must not change an existing inherited constraint to "no inherit"
+		 * status.  That's because inherited constraints should be able to
+		 * propagate to lower-level children.
+		 */
+		if (con->coninhcount > 0 && is_no_inherit)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+					 errmsg("constraint \"%s\" conflicts with inherited constraint on relation \"%s\"",
+							ccname, RelationGetRelationName(rel))));
 
-			/*
-			 * If the child constraint is "not valid" then cannot merge with a
-			 * valid parent constraint
-			 */
-			if (is_initially_valid && !con->convalidated)
-				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
-						 errmsg("constraint \"%s\" conflicts with NOT VALID constraint on relation \"%s\"",
-								ccname, RelationGetRelationName(rel))));
+		/*
+		 * If the child constraint is "not valid" then cannot merge with a
+		 * valid parent constraint.
+		 */
+		if (is_initially_valid && !con->convalidated)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+					 errmsg("constraint \"%s\" conflicts with NOT VALID constraint on relation \"%s\"",
+							ccname, RelationGetRelationName(rel))));
 
-			/* OK to update the tuple */
-			ereport(NOTICE,
-					(errmsg("merging constraint \"%s\" with inherited definition",
-							ccname)));
+		/* OK to update the tuple */
+		ereport(NOTICE,
+				(errmsg("merging constraint \"%s\" with inherited definition",
+						ccname)));
 
-			tup = heap_copytuple(tup);
-			con = (Form_pg_constraint) GETSTRUCT(tup);
+		tup = heap_copytuple(tup);
+		con = (Form_pg_constraint) GETSTRUCT(tup);
 
-			/*
-			 * In case of partitions, an inherited constraint must be
-			 * inherited only once since it cannot have multiple parents and
-			 * it is never considered local.
-			 */
-			if (rel->rd_rel->relispartition)
-			{
-				con->coninhcount = 1;
-				con->conislocal = false;
-			}
+		/*
+		 * In case of partitions, an inherited constraint must be inherited
+		 * only once since it cannot have multiple parents and it is never
+		 * considered local.
+		 */
+		if (rel->rd_rel->relispartition)
+		{
+			con->coninhcount = 1;
+			con->conislocal = false;
+		}
+		else
+		{
+			if (is_local)
+				con->conislocal = true;
 			else
-			{
-				if (is_local)
-					con->conislocal = true;
-				else
-					con->coninhcount++;
-			}
+				con->coninhcount++;
+		}
 
-			if (is_no_inherit)
-			{
-				Assert(is_local);
-				con->connoinherit = true;
-			}
-			CatalogTupleUpdate(conDesc, &tup->t_self, tup);
-			break;
+		if (is_no_inherit)
+		{
+			Assert(is_local);
+			con->connoinherit = true;
 		}
+
+		CatalogTupleUpdate(conDesc, &tup->t_self, tup);
 	}
 
 	systable_endscan(conscan);
diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index b2560549083..4b29120f193 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -842,6 +842,12 @@ index_create(Relation heapRelation,
 	if (shared_relation && tableSpaceId != GLOBALTABLESPACE_OID)
 		elog(ERROR, "shared relations must be placed in pg_global tablespace");
 
+	/*
+	 * Check for duplicate name (both as to the index, and as to the
+	 * associated constraint if any).  Such cases would fail on the relevant
+	 * catalogs' unique indexes anyway, but we prefer to give a friendlier
+	 * error message.
+	 */
 	if (get_relname_relid(indexRelationName, namespaceId))
 	{
 		if ((flags & INDEX_CREATE_IF_NOT_EXISTS) != 0)
@@ -860,6 +866,20 @@ index_create(Relation heapRelation,
 						indexRelationName)));
 	}
 
+	if ((flags & INDEX_CREATE_ADD_CONSTRAINT) != 0 &&
+		ConstraintNameIsUsed(CONSTRAINT_RELATION, heapRelationId,
+							 indexRelationName))
+	{
+		/*
+		 * INDEX_CREATE_IF_NOT_EXISTS does not apply here, since the
+		 * conflicting constraint is not an index.
+		 */
+		ereport(ERROR,
+				(errcode(ERRCODE_DUPLICATE_OBJECT),
+				 errmsg("constraint \"%s\" for relation \"%s\" already exists",
+						indexRelationName, RelationGetRelationName(heapRelation))));
+	}
+
 	/*
 	 * construct tuple descriptor for index tuples
 	 */
diff --git a/src/backend/catalog/pg_constraint.c b/src/backend/catalog/pg_constraint.c
index ea844413608..6781b00c6e6 100644
--- a/src/backend/catalog/pg_constraint.c
+++ b/src/backend/catalog/pg_constraint.c
@@ -422,7 +422,7 @@ CloneForeignKeyConstraints(Oid parentId, Oid relationId, List **cloned)
 	ScanKeyInit(&key,
 				Anum_pg_constraint_conrelid, BTEqualStrategyNumber,
 				F_OIDEQ, ObjectIdGetDatum(parentId));
-	scan = systable_beginscan(pg_constraint, ConstraintRelidIndexId, true,
+	scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, true,
 							  NULL, 1, &key);
 
 	while ((tuple = systable_getnext(scan)) != NULL)
@@ -632,17 +632,58 @@ CloneForeignKeyConstraints(Oid parentId, Oid relationId, List **cloned)
  */
 bool
 ConstraintNameIsUsed(ConstraintCategory conCat, Oid objId,
-					 Oid objNamespace, const char *conname)
+					 const char *conname)
 {
 	bool		found;
 	Relation	conDesc;
 	SysScanDesc conscan;
-	ScanKeyData skey[2];
-	HeapTuple	tup;
+	ScanKeyData skey[3];
 
 	conDesc = heap_open(ConstraintRelationId, AccessShareLock);
 
-	found = false;
+	ScanKeyInit(&skey[0],
+				Anum_pg_constraint_conrelid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum((conCat == CONSTRAINT_RELATION)
+								 ? objId : InvalidOid));
+	ScanKeyInit(&skey[1],
+				Anum_pg_constraint_contypid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum((conCat == CONSTRAINT_DOMAIN)
+								 ? objId : InvalidOid));
+	ScanKeyInit(&skey[2],
+				Anum_pg_constraint_conname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(conname));
+
+	conscan = systable_beginscan(conDesc, ConstraintRelidTypidNameIndexId,
+								 true, NULL, 3, skey);
+
+	/* There can be at most one matching row */
+	found = (HeapTupleIsValid(systable_getnext(conscan)));
+
+	systable_endscan(conscan);
+	heap_close(conDesc, AccessShareLock);
+
+	return found;
+}
+
+/*
+ * Does any constraint of the given name exist in the given namespace?
+ *
+ * This is used for code that wants to match ChooseConstraintName's rule
+ * that we should avoid autogenerating duplicate constraint names within a
+ * namespace.
+ */
+bool
+ConstraintNameExists(const char *conname, Oid namespaceid)
+{
+	bool		found;
+	Relation	conDesc;
+	SysScanDesc conscan;
+	ScanKeyData skey[2];
+
+	conDesc = heap_open(ConstraintRelationId, AccessShareLock);
 
 	ScanKeyInit(&skey[0],
 				Anum_pg_constraint_conname,
@@ -652,26 +693,12 @@ ConstraintNameIsUsed(ConstraintCategory conCat, Oid objId,
 	ScanKeyInit(&skey[1],
 				Anum_pg_constraint_connamespace,
 				BTEqualStrategyNumber, F_OIDEQ,
-				ObjectIdGetDatum(objNamespace));
+				ObjectIdGetDatum(namespaceid));
 
 	conscan = systable_beginscan(conDesc, ConstraintNameNspIndexId, true,
 								 NULL, 2, skey);
 
-	while (HeapTupleIsValid(tup = systable_getnext(conscan)))
-	{
-		Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(tup);
-
-		if (conCat == CONSTRAINT_RELATION && con->conrelid == objId)
-		{
-			found = true;
-			break;
-		}
-		else if (conCat == CONSTRAINT_DOMAIN && con->contypid == objId)
-		{
-			found = true;
-			break;
-		}
-	}
+	found = (HeapTupleIsValid(systable_getnext(conscan)));
 
 	systable_endscan(conscan);
 	heap_close(conDesc, AccessShareLock);
@@ -878,13 +905,11 @@ RenameConstraintById(Oid conId, const char *newname)
 	con = (Form_pg_constraint) GETSTRUCT(tuple);
 
 	/*
-	 * We need to check whether the name is already in use --- note that there
-	 * currently is not a unique index that would catch this.
+	 * For user-friendliness, check whether the name is already in use.
 	 */
 	if (OidIsValid(con->conrelid) &&
 		ConstraintNameIsUsed(CONSTRAINT_RELATION,
 							 con->conrelid,
-							 con->connamespace,
 							 newname))
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_OBJECT),
@@ -893,7 +918,6 @@ RenameConstraintById(Oid conId, const char *newname)
 	if (OidIsValid(con->contypid) &&
 		ConstraintNameIsUsed(CONSTRAINT_DOMAIN,
 							 con->contypid,
-							 con->connamespace,
 							 newname))
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_OBJECT),
@@ -923,32 +947,23 @@ AlterConstraintNamespaces(Oid ownerId, Oid oldNspId,
 						  Oid newNspId, bool isType, ObjectAddresses *objsMoved)
 {
 	Relation	conRel;
-	ScanKeyData key[1];
+	ScanKeyData key[2];
 	SysScanDesc scan;
 	HeapTuple	tup;
 
 	conRel = heap_open(ConstraintRelationId, RowExclusiveLock);
 
-	if (isType)
-	{
-		ScanKeyInit(&key[0],
-					Anum_pg_constraint_contypid,
-					BTEqualStrategyNumber, F_OIDEQ,
-					ObjectIdGetDatum(ownerId));
-
-		scan = systable_beginscan(conRel, ConstraintTypidIndexId, true,
-								  NULL, 1, key);
-	}
-	else
-	{
-		ScanKeyInit(&key[0],
-					Anum_pg_constraint_conrelid,
-					BTEqualStrategyNumber, F_OIDEQ,
-					ObjectIdGetDatum(ownerId));
+	ScanKeyInit(&key[0],
+				Anum_pg_constraint_conrelid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(isType ? InvalidOid : ownerId));
+	ScanKeyInit(&key[1],
+				Anum_pg_constraint_contypid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(isType ? ownerId : InvalidOid));
 
-		scan = systable_beginscan(conRel, ConstraintRelidIndexId, true,
-								  NULL, 1, key);
-	}
+	scan = systable_beginscan(conRel, ConstraintRelidTypidNameIndexId, true,
+							  NULL, 2, key);
 
 	while (HeapTupleIsValid((tup = systable_getnext(scan))))
 	{
@@ -1038,38 +1053,30 @@ get_relation_constraint_oid(Oid relid, const char *conname, bool missing_ok)
 	Relation	pg_constraint;
 	HeapTuple	tuple;
 	SysScanDesc scan;
-	ScanKeyData skey[1];
+	ScanKeyData skey[3];
 	Oid			conOid = InvalidOid;
 
-	/*
-	 * Fetch the constraint tuple from pg_constraint.  There may be more than
-	 * one match, because constraints are not required to have unique names;
-	 * if so, error out.
-	 */
 	pg_constraint = heap_open(ConstraintRelationId, AccessShareLock);
 
 	ScanKeyInit(&skey[0],
 				Anum_pg_constraint_conrelid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(relid));
+	ScanKeyInit(&skey[1],
+				Anum_pg_constraint_contypid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(InvalidOid));
+	ScanKeyInit(&skey[2],
+				Anum_pg_constraint_conname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(conname));
 
-	scan = systable_beginscan(pg_constraint, ConstraintRelidIndexId, true,
-							  NULL, 1, skey);
-
-	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
-	{
-		Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(tuple);
+	scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, true,
+							  NULL, 3, skey);
 
-		if (strcmp(NameStr(con->conname), conname) == 0)
-		{
-			if (OidIsValid(conOid))
-				ereport(ERROR,
-						(errcode(ERRCODE_DUPLICATE_OBJECT),
-						 errmsg("table \"%s\" has multiple constraints named \"%s\"",
-								get_rel_name(relid), conname)));
-			conOid = HeapTupleGetOid(tuple);
-		}
-	}
+	/* There can be at most one matching row */
+	if (HeapTupleIsValid(tuple = systable_getnext(scan)))
+		conOid = HeapTupleGetOid(tuple);
 
 	systable_endscan(scan);
 
@@ -1105,67 +1112,62 @@ get_relation_constraint_attnos(Oid relid, const char *conname,
 	Relation	pg_constraint;
 	HeapTuple	tuple;
 	SysScanDesc scan;
-	ScanKeyData skey[1];
+	ScanKeyData skey[3];
 
 	/* Set *constraintOid, to avoid complaints about uninitialized vars */
 	*constraintOid = InvalidOid;
 
-	/*
-	 * Fetch the constraint tuple from pg_constraint.  There may be more than
-	 * one match, because constraints are not required to have unique names;
-	 * if so, error out.
-	 */
 	pg_constraint = heap_open(ConstraintRelationId, AccessShareLock);
 
 	ScanKeyInit(&skey[0],
 				Anum_pg_constraint_conrelid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(relid));
+	ScanKeyInit(&skey[1],
+				Anum_pg_constraint_contypid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(InvalidOid));
+	ScanKeyInit(&skey[2],
+				Anum_pg_constraint_conname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(conname));
 
-	scan = systable_beginscan(pg_constraint, ConstraintRelidIndexId, true,
-							  NULL, 1, skey);
+	scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, true,
+							  NULL, 3, skey);
 
-	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
+	/* There can be at most one matching row */
+	if (HeapTupleIsValid(tuple = systable_getnext(scan)))
 	{
-		Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(tuple);
 		Datum		adatum;
 		bool		isNull;
-		ArrayType  *arr;
-		int16	   *attnums;
-		int			numcols;
-		int			i;
-
-		/* Check the constraint name */
-		if (strcmp(NameStr(con->conname), conname) != 0)
-			continue;
-		if (OidIsValid(*constraintOid))
-			ereport(ERROR,
-					(errcode(ERRCODE_DUPLICATE_OBJECT),
-					 errmsg("table \"%s\" has multiple constraints named \"%s\"",
-							get_rel_name(relid), conname)));
 
 		*constraintOid = HeapTupleGetOid(tuple);
 
 		/* Extract the conkey array, ie, attnums of constrained columns */
 		adatum = heap_getattr(tuple, Anum_pg_constraint_conkey,
 							  RelationGetDescr(pg_constraint), &isNull);
-		if (isNull)
-			continue;			/* no constrained columns */
-
-		arr = DatumGetArrayTypeP(adatum);	/* ensure not toasted */
-		numcols = ARR_DIMS(arr)[0];
-		if (ARR_NDIM(arr) != 1 ||
-			numcols < 0 ||
-			ARR_HASNULL(arr) ||
-			ARR_ELEMTYPE(arr) != INT2OID)
-			elog(ERROR, "conkey is not a 1-D smallint array");
-		attnums = (int16 *) ARR_DATA_PTR(arr);
-
-		/* Construct the result value */
-		for (i = 0; i < numcols; i++)
+		if (!isNull)
 		{
-			conattnos = bms_add_member(conattnos,
-									   attnums[i] - FirstLowInvalidHeapAttributeNumber);
+			ArrayType  *arr;
+			int			numcols;
+			int16	   *attnums;
+			int			i;
+
+			arr = DatumGetArrayTypeP(adatum);	/* ensure not toasted */
+			numcols = ARR_DIMS(arr)[0];
+			if (ARR_NDIM(arr) != 1 ||
+				numcols < 0 ||
+				ARR_HASNULL(arr) ||
+				ARR_ELEMTYPE(arr) != INT2OID)
+				elog(ERROR, "conkey is not a 1-D smallint array");
+			attnums = (int16 *) ARR_DATA_PTR(arr);
+
+			/* Construct the result value */
+			for (i = 0; i < numcols; i++)
+			{
+				conattnos = bms_add_member(conattnos,
+										   attnums[i] - FirstLowInvalidHeapAttributeNumber);
+			}
 		}
 	}
 
@@ -1203,7 +1205,7 @@ get_relation_idx_constraint_oid(Oid relationId, Oid indexId)
 				BTEqualStrategyNumber,
 				F_OIDEQ,
 				ObjectIdGetDatum(relationId));
-	scan = systable_beginscan(pg_constraint, ConstraintRelidIndexId,
+	scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId,
 							  true, NULL, 1, &key);
 	while ((tuple = systable_getnext(scan)) != NULL)
 	{
@@ -1233,38 +1235,30 @@ get_domain_constraint_oid(Oid typid, const char *conname, bool missing_ok)
 	Relation	pg_constraint;
 	HeapTuple	tuple;
 	SysScanDesc scan;
-	ScanKeyData skey[1];
+	ScanKeyData skey[3];
 	Oid			conOid = InvalidOid;
 
-	/*
-	 * Fetch the constraint tuple from pg_constraint.  There may be more than
-	 * one match, because constraints are not required to have unique names;
-	 * if so, error out.
-	 */
 	pg_constraint = heap_open(ConstraintRelationId, AccessShareLock);
 
 	ScanKeyInit(&skey[0],
+				Anum_pg_constraint_conrelid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(InvalidOid));
+	ScanKeyInit(&skey[1],
 				Anum_pg_constraint_contypid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(typid));
+	ScanKeyInit(&skey[2],
+				Anum_pg_constraint_conname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(conname));
 
-	scan = systable_beginscan(pg_constraint, ConstraintTypidIndexId, true,
-							  NULL, 1, skey);
-
-	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
-	{
-		Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(tuple);
+	scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, true,
+							  NULL, 3, skey);
 
-		if (strcmp(NameStr(con->conname), conname) == 0)
-		{
-			if (OidIsValid(conOid))
-				ereport(ERROR,
-						(errcode(ERRCODE_DUPLICATE_OBJECT),
-						 errmsg("domain %s has multiple constraints named \"%s\"",
-								format_type_be(typid), conname)));
-			conOid = HeapTupleGetOid(tuple);
-		}
-	}
+	/* There can be at most one matching row */
+	if (HeapTupleIsValid(tuple = systable_getnext(scan)))
+		conOid = HeapTupleGetOid(tuple);
 
 	systable_endscan(scan);
 
@@ -1314,7 +1308,7 @@ get_primary_key_attnos(Oid relid, bool deferrableOk, Oid *constraintOid)
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(relid));
 
-	scan = systable_beginscan(pg_constraint, ConstraintRelidIndexId, true,
+	scan = systable_beginscan(pg_constraint, ConstraintRelidTypidNameIndexId, true,
 							  NULL, 1, skey);
 
 	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index d54c78c3527..ab3d9a0a489 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -1995,6 +1995,12 @@ makeObjectName(const char *name1, const char *name2, const char *label)
  * except that the label can't be NULL; digits will be appended to the label
  * if needed to create a name that is unique within the specified namespace.
  *
+ * If isconstraint is true, we also avoid choosing a name matching any
+ * existing constraint in the same namespace.  (This is stricter than what
+ * Postgres itself requires, but the SQL standard says that constraint names
+ * should be unique within schemas, so we follow that for autogenerated
+ * constraint names.)
+ *
  * Note: it is theoretically possible to get a collision anyway, if someone
  * else chooses the same name concurrently.  This is fairly unlikely to be
  * a problem in practice, especially if one is holding an exclusive lock on
@@ -2006,7 +2012,8 @@ makeObjectName(const char *name1, const char *name2, const char *label)
  */
 char *
 ChooseRelationName(const char *name1, const char *name2,
-				   const char *label, Oid namespaceid)
+				   const char *label, Oid namespaceid,
+				   bool isconstraint)
 {
 	int			pass = 0;
 	char	   *relname = NULL;
@@ -2020,7 +2027,11 @@ ChooseRelationName(const char *name1, const char *name2,
 		relname = makeObjectName(name1, name2, modlabel);
 
 		if (!OidIsValid(get_relname_relid(relname, namespaceid)))
-			break;
+		{
+			if (!isconstraint ||
+				!ConstraintNameExists(relname, namespaceid))
+				break;
+		}
 
 		/* found a conflict, so try a new name component */
 		pfree(relname);
@@ -2048,28 +2059,32 @@ ChooseIndexName(const char *tabname, Oid namespaceId,
 		indexname = ChooseRelationName(tabname,
 									   NULL,
 									   "pkey",
-									   namespaceId);
+									   namespaceId,
+									   true);
 	}
 	else if (exclusionOpNames != NIL)
 	{
 		indexname = ChooseRelationName(tabname,
 									   ChooseIndexNameAddition(colnames),
 									   "excl",
-									   namespaceId);
+									   namespaceId,
+									   true);
 	}
 	else if (isconstraint)
 	{
 		indexname = ChooseRelationName(tabname,
 									   ChooseIndexNameAddition(colnames),
 									   "key",
-									   namespaceId);
+									   namespaceId,
+									   true);
 	}
 	else
 	{
 		indexname = ChooseRelationName(tabname,
 									   ChooseIndexNameAddition(colnames),
 									   "idx",
-									   namespaceId);
+									   namespaceId,
+									   false);
 	}
 
 	return indexname;
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index f46af41b562..f9e83c24565 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -7145,7 +7145,6 @@ ATExecAddConstraint(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			{
 				if (ConstraintNameIsUsed(CONSTRAINT_RELATION,
 										 RelationGetRelid(rel),
-										 RelationGetNamespace(rel),
 										 newConstraint->conname))
 					ereport(ERROR,
 							(errcode(ERRCODE_DUPLICATE_OBJECT),
@@ -7801,10 +7800,9 @@ ATExecAlterConstraint(Relation rel, AlterTableCmd *cmd,
 	Constraint *cmdcon;
 	Relation	conrel;
 	SysScanDesc scan;
-	ScanKeyData key;
+	ScanKeyData skey[3];
 	HeapTuple	contuple;
-	Form_pg_constraint currcon = NULL;
-	bool		found = false;
+	Form_pg_constraint currcon;
 	ObjectAddress address;
 
 	cmdcon = castNode(Constraint, cmd->def);
@@ -7814,29 +7812,29 @@ ATExecAlterConstraint(Relation rel, AlterTableCmd *cmd,
 	/*
 	 * Find and check the target constraint
 	 */
-	ScanKeyInit(&key,
+	ScanKeyInit(&skey[0],
 				Anum_pg_constraint_conrelid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(RelationGetRelid(rel)));
-	scan = systable_beginscan(conrel, ConstraintRelidIndexId,
-							  true, NULL, 1, &key);
-
-	while (HeapTupleIsValid(contuple = systable_getnext(scan)))
-	{
-		currcon = (Form_pg_constraint) GETSTRUCT(contuple);
-		if (strcmp(NameStr(currcon->conname), cmdcon->conname) == 0)
-		{
-			found = true;
-			break;
-		}
-	}
+	ScanKeyInit(&skey[1],
+				Anum_pg_constraint_contypid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(InvalidOid));
+	ScanKeyInit(&skey[2],
+				Anum_pg_constraint_conname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(cmdcon->conname));
+	scan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId,
+							  true, NULL, 3, skey);
 
-	if (!found)
+	/* There can be at most one matching row */
+	if (!HeapTupleIsValid(contuple = systable_getnext(scan)))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("constraint \"%s\" of relation \"%s\" does not exist",
 						cmdcon->conname, RelationGetRelationName(rel))));
 
+	currcon = (Form_pg_constraint) GETSTRUCT(contuple);
 	if (currcon->contype != CONSTRAINT_FOREIGN)
 		ereport(ERROR,
 				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
@@ -7969,10 +7967,9 @@ ATExecValidateConstraint(Relation rel, char *constrName, bool recurse,
 {
 	Relation	conrel;
 	SysScanDesc scan;
-	ScanKeyData key;
+	ScanKeyData skey[3];
 	HeapTuple	tuple;
-	Form_pg_constraint con = NULL;
-	bool		found = false;
+	Form_pg_constraint con;
 	ObjectAddress address;
 
 	conrel = heap_open(ConstraintRelationId, RowExclusiveLock);
@@ -7980,29 +7977,29 @@ ATExecValidateConstraint(Relation rel, char *constrName, bool recurse,
 	/*
 	 * Find and check the target constraint
 	 */
-	ScanKeyInit(&key,
+	ScanKeyInit(&skey[0],
 				Anum_pg_constraint_conrelid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(RelationGetRelid(rel)));
-	scan = systable_beginscan(conrel, ConstraintRelidIndexId,
-							  true, NULL, 1, &key);
-
-	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
-	{
-		con = (Form_pg_constraint) GETSTRUCT(tuple);
-		if (strcmp(NameStr(con->conname), constrName) == 0)
-		{
-			found = true;
-			break;
-		}
-	}
+	ScanKeyInit(&skey[1],
+				Anum_pg_constraint_contypid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(InvalidOid));
+	ScanKeyInit(&skey[2],
+				Anum_pg_constraint_conname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(constrName));
+	scan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId,
+							  true, NULL, 3, skey);
 
-	if (!found)
+	/* There can be at most one matching row */
+	if (!HeapTupleIsValid(tuple = systable_getnext(scan)))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("constraint \"%s\" of relation \"%s\" does not exist",
 						constrName, RelationGetRelationName(rel))));
 
+	con = (Form_pg_constraint) GETSTRUCT(tuple);
 	if (con->contype != CONSTRAINT_FOREIGN &&
 		con->contype != CONSTRAINT_CHECK)
 		ereport(ERROR,
@@ -8865,7 +8862,7 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 	Relation	conrel;
 	Form_pg_constraint con;
 	SysScanDesc scan;
-	ScanKeyData key;
+	ScanKeyData skey[3];
 	HeapTuple	tuple;
 	bool		found = false;
 	bool		is_no_inherit_constraint = false;
@@ -8879,22 +8876,28 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 	/*
 	 * Find and drop the target constraint
 	 */
-	ScanKeyInit(&key,
+	ScanKeyInit(&skey[0],
 				Anum_pg_constraint_conrelid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(RelationGetRelid(rel)));
-	scan = systable_beginscan(conrel, ConstraintRelidIndexId,
-							  true, NULL, 1, &key);
+	ScanKeyInit(&skey[1],
+				Anum_pg_constraint_contypid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(InvalidOid));
+	ScanKeyInit(&skey[2],
+				Anum_pg_constraint_conname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(constrName));
+	scan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId,
+							  true, NULL, 3, skey);
 
-	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
+	/* There can be at most one matching row */
+	if (HeapTupleIsValid(tuple = systable_getnext(scan)))
 	{
 		ObjectAddress conobj;
 
 		con = (Form_pg_constraint) GETSTRUCT(tuple);
 
-		if (strcmp(NameStr(con->conname), constrName) != 0)
-			continue;
-
 		/* Don't drop inherited constraints */
 		if (con->coninhcount > 0 && !recursing)
 			ereport(ERROR,
@@ -8932,9 +8935,6 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 		performDeletion(&conobj, behavior, 0);
 
 		found = true;
-
-		/* constraint found and dropped -- no need to keep looping */
-		break;
 	}
 
 	systable_endscan(scan);
@@ -8990,27 +8990,23 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 		childrel = heap_open(childrelid, NoLock);
 		CheckTableNotInUse(childrel, "ALTER TABLE");
 
-		ScanKeyInit(&key,
+		ScanKeyInit(&skey[0],
 					Anum_pg_constraint_conrelid,
 					BTEqualStrategyNumber, F_OIDEQ,
 					ObjectIdGetDatum(childrelid));
-		scan = systable_beginscan(conrel, ConstraintRelidIndexId,
-								  true, NULL, 1, &key);
-
-		/* scan for matching tuple - there should only be one */
-		while (HeapTupleIsValid(tuple = systable_getnext(scan)))
-		{
-			con = (Form_pg_constraint) GETSTRUCT(tuple);
-
-			/* Right now only CHECK constraints can be inherited */
-			if (con->contype != CONSTRAINT_CHECK)
-				continue;
-
-			if (strcmp(NameStr(con->conname), constrName) == 0)
-				break;
-		}
-
-		if (!HeapTupleIsValid(tuple))
+		ScanKeyInit(&skey[1],
+					Anum_pg_constraint_contypid,
+					BTEqualStrategyNumber, F_OIDEQ,
+					ObjectIdGetDatum(InvalidOid));
+		ScanKeyInit(&skey[2],
+					Anum_pg_constraint_conname,
+					BTEqualStrategyNumber, F_NAMEEQ,
+					CStringGetDatum(constrName));
+		scan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId,
+								  true, NULL, 3, skey);
+
+		/* There can be at most one matching row */
+		if (!HeapTupleIsValid(tuple = systable_getnext(scan)))
 			ereport(ERROR,
 					(errcode(ERRCODE_UNDEFINED_OBJECT),
 					 errmsg("constraint \"%s\" of relation \"%s\" does not exist",
@@ -9023,6 +9019,10 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 
 		con = (Form_pg_constraint) GETSTRUCT(copy_tuple);
 
+		/* Right now only CHECK constraints can be inherited */
+		if (con->contype != CONSTRAINT_CHECK)
+			elog(ERROR, "inherited constraint is not a CHECK constraint");
+
 		if (con->coninhcount <= 0)	/* shouldn't happen */
 			elog(ERROR, "relation %u has non-inherited constraint \"%s\"",
 				 childrelid, constrName);
@@ -11824,7 +11824,7 @@ MergeConstraintsIntoExisting(Relation child_rel, Relation parent_rel)
 				Anum_pg_constraint_conrelid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(RelationGetRelid(parent_rel)));
-	parent_scan = systable_beginscan(catalog_relation, ConstraintRelidIndexId,
+	parent_scan = systable_beginscan(catalog_relation, ConstraintRelidTypidNameIndexId,
 									 true, NULL, 1, &parent_key);
 
 	while (HeapTupleIsValid(parent_tuple = systable_getnext(parent_scan)))
@@ -11847,7 +11847,7 @@ MergeConstraintsIntoExisting(Relation child_rel, Relation parent_rel)
 					Anum_pg_constraint_conrelid,
 					BTEqualStrategyNumber, F_OIDEQ,
 					ObjectIdGetDatum(RelationGetRelid(child_rel)));
-		child_scan = systable_beginscan(catalog_relation, ConstraintRelidIndexId,
+		child_scan = systable_beginscan(catalog_relation, ConstraintRelidTypidNameIndexId,
 										true, NULL, 1, &child_key);
 
 		while (HeapTupleIsValid(child_tuple = systable_getnext(child_scan)))
@@ -12068,7 +12068,7 @@ RemoveInheritance(Relation child_rel, Relation parent_rel)
 				Anum_pg_constraint_conrelid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(RelationGetRelid(parent_rel)));
-	scan = systable_beginscan(catalogRelation, ConstraintRelidIndexId,
+	scan = systable_beginscan(catalogRelation, ConstraintRelidTypidNameIndexId,
 							  true, NULL, 1, key);
 
 	connames = NIL;
@@ -12088,7 +12088,7 @@ RemoveInheritance(Relation child_rel, Relation parent_rel)
 				Anum_pg_constraint_conrelid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(RelationGetRelid(child_rel)));
-	scan = systable_beginscan(catalogRelation, ConstraintRelidIndexId,
+	scan = systable_beginscan(catalogRelation, ConstraintRelidTypidNameIndexId,
 							  true, NULL, 1, key);
 
 	while (HeapTupleIsValid(constraintTuple = systable_getnext(scan)))
@@ -12829,7 +12829,7 @@ ATPrepChangePersistence(Relation rel, bool toLogged)
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(RelationGetRelid(rel)));
 	scan = systable_beginscan(pg_constraint,
-							  toLogged ? ConstraintRelidIndexId : InvalidOid,
+							  toLogged ? ConstraintRelidTypidNameIndexId : InvalidOid,
 							  true, NULL, 1, skey);
 
 	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
diff --git a/src/backend/commands/typecmds.c b/src/backend/commands/typecmds.c
index 175ecc8b484..b018585aef8 100644
--- a/src/backend/commands/typecmds.c
+++ b/src/backend/commands/typecmds.c
@@ -2444,6 +2444,8 @@ AlterDomainNotNull(List *names, bool notNull)
  * AlterDomainDropConstraint
  *
  * Implements the ALTER DOMAIN DROP CONSTRAINT statement
+ *
+ * Returns ObjectAddress of the modified domain.
  */
 ObjectAddress
 AlterDomainDropConstraint(List *names, const char *constrName,
@@ -2455,10 +2457,10 @@ AlterDomainDropConstraint(List *names, const char *constrName,
 	Relation	rel;
 	Relation	conrel;
 	SysScanDesc conscan;
-	ScanKeyData key[1];
+	ScanKeyData skey[3];
 	HeapTuple	contup;
 	bool		found = false;
-	ObjectAddress address = InvalidObjectAddress;
+	ObjectAddress address;
 
 	/* Make a TypeName so we can use standard type lookup machinery */
 	typename = makeTypeNameFromNameList(names);
@@ -2477,37 +2479,36 @@ AlterDomainDropConstraint(List *names, const char *constrName,
 	/* Grab an appropriate lock on the pg_constraint relation */
 	conrel = heap_open(ConstraintRelationId, RowExclusiveLock);
 
-	/* Use the index to scan only constraints of the target relation */
-	ScanKeyInit(&key[0],
+	/* Find and remove the target constraint */
+	ScanKeyInit(&skey[0],
+				Anum_pg_constraint_conrelid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(InvalidOid));
+	ScanKeyInit(&skey[1],
 				Anum_pg_constraint_contypid,
 				BTEqualStrategyNumber, F_OIDEQ,
-				ObjectIdGetDatum(HeapTupleGetOid(tup)));
+				ObjectIdGetDatum(domainoid));
+	ScanKeyInit(&skey[2],
+				Anum_pg_constraint_conname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(constrName));
 
-	conscan = systable_beginscan(conrel, ConstraintTypidIndexId, true,
-								 NULL, 1, key);
+	conscan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId, true,
+								 NULL, 3, skey);
 
-	/*
-	 * Scan over the result set, removing any matching entries.
-	 */
-	while ((contup = systable_getnext(conscan)) != NULL)
+	/* There can be at most one matching row */
+	if ((contup = systable_getnext(conscan)) != NULL)
 	{
-		Form_pg_constraint con = (Form_pg_constraint) GETSTRUCT(contup);
-
-		if (strcmp(NameStr(con->conname), constrName) == 0)
-		{
-			ObjectAddress conobj;
+		ObjectAddress conobj;
 
-			conobj.classId = ConstraintRelationId;
-			conobj.objectId = HeapTupleGetOid(contup);
-			conobj.objectSubId = 0;
+		conobj.classId = ConstraintRelationId;
+		conobj.objectId = HeapTupleGetOid(contup);
+		conobj.objectSubId = 0;
 
-			performDeletion(&conobj, behavior, 0);
-			found = true;
-		}
+		performDeletion(&conobj, behavior, 0);
+		found = true;
 	}
 
-	ObjectAddressSet(address, TypeRelationId, domainoid);
-
 	/* Clean up after the scan */
 	systable_endscan(conscan);
 	heap_close(conrel, RowExclusiveLock);
@@ -2527,6 +2528,8 @@ AlterDomainDropConstraint(List *names, const char *constrName,
 							constrName, TypeNameToString(typename))));
 	}
 
+	ObjectAddressSet(address, TypeRelationId, domainoid);
+
 	return address;
 }
 
@@ -2652,16 +2655,15 @@ AlterDomainValidateConstraint(List *names, const char *constrName)
 	Relation	typrel;
 	Relation	conrel;
 	HeapTuple	tup;
-	Form_pg_constraint con = NULL;
+	Form_pg_constraint con;
 	Form_pg_constraint copy_con;
 	char	   *conbin;
 	SysScanDesc scan;
 	Datum		val;
-	bool		found = false;
 	bool		isnull;
 	HeapTuple	tuple;
 	HeapTuple	copyTuple;
-	ScanKeyData key;
+	ScanKeyData skey[3];
 	ObjectAddress address;
 
 	/* Make a TypeName so we can use standard type lookup machinery */
@@ -2682,29 +2684,31 @@ AlterDomainValidateConstraint(List *names, const char *constrName)
 	 * Find and check the target constraint
 	 */
 	conrel = heap_open(ConstraintRelationId, RowExclusiveLock);
-	ScanKeyInit(&key,
+
+	ScanKeyInit(&skey[0],
+				Anum_pg_constraint_conrelid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(InvalidOid));
+	ScanKeyInit(&skey[1],
 				Anum_pg_constraint_contypid,
 				BTEqualStrategyNumber, F_OIDEQ,
 				ObjectIdGetDatum(domainoid));
-	scan = systable_beginscan(conrel, ConstraintTypidIndexId,
-							  true, NULL, 1, &key);
+	ScanKeyInit(&skey[2],
+				Anum_pg_constraint_conname,
+				BTEqualStrategyNumber, F_NAMEEQ,
+				CStringGetDatum(constrName));
 
-	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
-	{
-		con = (Form_pg_constraint) GETSTRUCT(tuple);
-		if (strcmp(NameStr(con->conname), constrName) == 0)
-		{
-			found = true;
-			break;
-		}
-	}
+	scan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId, true,
+							  NULL, 3, skey);
 
-	if (!found)
+	/* There can be at most one matching row */
+	if (!HeapTupleIsValid(tuple = systable_getnext(scan)))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("constraint \"%s\" of domain \"%s\" does not exist",
 						constrName, TypeNameToString(typename))));
 
+	con = (Form_pg_constraint) GETSTRUCT(tuple);
 	if (con->contype != CONSTRAINT_CHECK)
 		ereport(ERROR,
 				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
@@ -3072,7 +3076,6 @@ domainAddConstraint(Oid domainOid, Oid domainNamespace, Oid baseTypeOid,
 	{
 		if (ConstraintNameIsUsed(CONSTRAINT_DOMAIN,
 								 domainOid,
-								 domainNamespace,
 								 constr->conname))
 			ereport(ERROR,
 					(errcode(ERRCODE_DUPLICATE_OBJECT),
diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index 656b1b5f1b4..f5c1e2a0d73 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -440,7 +440,8 @@ generateSerialExtraStmts(CreateStmtContext *cxt, ColumnDef *column,
 		sname = ChooseRelationName(cxt->relation->relname,
 								   column->colname,
 								   "seq",
-								   snamespaceid);
+								   snamespaceid,
+								   false);
 	}
 
 	ereport(DEBUG1,
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 6125421d39a..a4fc0011031 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -4016,7 +4016,7 @@ CheckConstraintFetch(Relation relation)
 				ObjectIdGetDatum(RelationGetRelid(relation)));
 
 	conrel = heap_open(ConstraintRelationId, AccessShareLock);
-	conscan = systable_beginscan(conrel, ConstraintRelidIndexId, true,
+	conscan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId, true,
 								 NULL, 1, skey);
 
 	while (HeapTupleIsValid(htup = systable_getnext(conscan)))
@@ -4127,7 +4127,7 @@ RelationGetFKeyList(Relation relation)
 				ObjectIdGetDatum(RelationGetRelid(relation)));
 
 	conrel = heap_open(ConstraintRelationId, AccessShareLock);
-	conscan = systable_beginscan(conrel, ConstraintRelidIndexId, true,
+	conscan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId, true,
 								 NULL, 1, &skey);
 
 	while (HeapTupleIsValid(htup = systable_getnext(conscan)))
@@ -5105,6 +5105,10 @@ RelationGetExclusionInfo(Relation indexRelation,
 	 * Search pg_constraint for the constraint associated with the index. To
 	 * make this not too painfully slow, we use the index on conrelid; that
 	 * will hold the parent relation's OID not the index's own OID.
+	 *
+	 * Note: if we wanted to rely on the constraint name matching the index's
+	 * name, we could just do a direct lookup using pg_constraint's unique
+	 * index.  For the moment it doesn't seem worth requiring that.
 	 */
 	ScanKeyInit(&skey[0],
 				Anum_pg_constraint_conrelid,
@@ -5112,7 +5116,7 @@ RelationGetExclusionInfo(Relation indexRelation,
 				ObjectIdGetDatum(indexRelation->rd_index->indrelid));
 
 	conrel = heap_open(ConstraintRelationId, AccessShareLock);
-	conscan = systable_beginscan(conrel, ConstraintRelidIndexId, true,
+	conscan = systable_beginscan(conrel, ConstraintRelidTypidNameIndexId, true,
 								 NULL, 1, skey);
 	found = false;
 
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
index 0585e9b3bad..a365807dab1 100644
--- a/src/include/catalog/catversion.h
+++ b/src/include/catalog/catversion.h
@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201809031
+#define CATALOG_VERSION_NO	201809042
 
 #endif
diff --git a/src/include/catalog/indexing.h b/src/include/catalog/indexing.h
index 24915824caa..254fbef1f78 100644
--- a/src/include/catalog/indexing.h
+++ b/src/include/catalog/indexing.h
@@ -121,8 +121,8 @@ DECLARE_UNIQUE_INDEX(pg_collation_oid_index, 3085, on pg_collation using btree(o
 
 DECLARE_INDEX(pg_constraint_conname_nsp_index, 2664, on pg_constraint using btree(conname name_ops, connamespace oid_ops));
 #define ConstraintNameNspIndexId  2664
-DECLARE_INDEX(pg_constraint_conrelid_index, 2665, on pg_constraint using btree(conrelid oid_ops));
-#define ConstraintRelidIndexId	2665
+DECLARE_UNIQUE_INDEX(pg_constraint_conrelid_contypid_conname_index, 2665, on pg_constraint using btree(conrelid oid_ops, contypid oid_ops, conname name_ops));
+#define ConstraintRelidTypidNameIndexId	2665
 DECLARE_INDEX(pg_constraint_contypid_index, 2666, on pg_constraint using btree(contypid oid_ops));
 #define ConstraintTypidIndexId	2666
 DECLARE_UNIQUE_INDEX(pg_constraint_oid_index, 2667, on pg_constraint using btree(oid oid_ops));
diff --git a/src/include/catalog/pg_constraint.h b/src/include/catalog/pg_constraint.h
index 9d209c9d190..66b3f13f74a 100644
--- a/src/include/catalog/pg_constraint.h
+++ b/src/include/catalog/pg_constraint.h
@@ -39,6 +39,10 @@ CATALOG(pg_constraint,2606,ConstraintRelationId)
 	 * global lock to generate a globally unique name for a nameless
 	 * constraint.  We associate a namespace with constraint names only for
 	 * SQL-spec compatibility.
+	 *
+	 * However, we do require conname to be unique among the constraints of a
+	 * single relation or domain.  This is enforced by a unique index on
+	 * conrelid + contypid + conname.
 	 */
 	NameData	conname;		/* name of this constraint */
 	Oid			connamespace;	/* OID of namespace containing constraint */
@@ -233,7 +237,8 @@ extern void RemoveConstraintById(Oid conId);
 extern void RenameConstraintById(Oid conId, const char *newname);
 
 extern bool ConstraintNameIsUsed(ConstraintCategory conCat, Oid objId,
-					 Oid objNamespace, const char *conname);
+					 const char *conname);
+extern bool ConstraintNameExists(const char *conname, Oid namespaceid);
 extern char *ChooseConstraintName(const char *name1, const char *name2,
 					 const char *label, Oid namespaceid,
 					 List *others);
diff --git a/src/include/commands/defrem.h b/src/include/commands/defrem.h
index 6b837236d4d..1d05a4bcdc8 100644
--- a/src/include/commands/defrem.h
+++ b/src/include/commands/defrem.h
@@ -41,7 +41,8 @@ extern void ReindexMultipleTables(const char *objectName, ReindexObjectType obje
 extern char *makeObjectName(const char *name1, const char *name2,
 			   const char *label);
 extern char *ChooseRelationName(const char *name1, const char *name2,
-				   const char *label, Oid namespaceid);
+				   const char *label, Oid namespaceid,
+				   bool isconstraint);
 extern bool CheckIndexCompatible(Oid oldId,
 					 const char *accessMethodName,
 					 List *attributeList,
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index 0218c2c3620..dccc9b27c59 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -2994,6 +2994,41 @@ Check constraints:
 
 DROP TABLE alter2.tt8;
 DROP SCHEMA alter2;
+--
+-- Check conflicts between index and CHECK constraint names
+--
+CREATE TABLE tt9(c integer);
+ALTER TABLE tt9 ADD CHECK(c > 1);
+ALTER TABLE tt9 ADD CHECK(c > 2);  -- picks nonconflicting name
+ALTER TABLE tt9 ADD CONSTRAINT foo CHECK(c > 3);
+ALTER TABLE tt9 ADD CONSTRAINT foo CHECK(c > 4);  -- fail, dup name
+ERROR:  constraint "foo" for relation "tt9" already exists
+ALTER TABLE tt9 ADD UNIQUE(c);
+ALTER TABLE tt9 ADD UNIQUE(c);  -- picks nonconflicting name
+ALTER TABLE tt9 ADD CONSTRAINT tt9_c_key UNIQUE(c);  -- fail, dup name
+ERROR:  relation "tt9_c_key" already exists
+ALTER TABLE tt9 ADD CONSTRAINT foo UNIQUE(c);  -- fail, dup name
+ERROR:  constraint "foo" for relation "tt9" already exists
+ALTER TABLE tt9 ADD CONSTRAINT tt9_c_key CHECK(c > 5);  -- fail, dup name
+ERROR:  constraint "tt9_c_key" for relation "tt9" already exists
+ALTER TABLE tt9 ADD CONSTRAINT tt9_c_key2 CHECK(c > 6);
+ALTER TABLE tt9 ADD UNIQUE(c);  -- picks nonconflicting name
+\d tt9
+                Table "public.tt9"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ c      | integer |           |          | 
+Indexes:
+    "tt9_c_key" UNIQUE CONSTRAINT, btree (c)
+    "tt9_c_key1" UNIQUE CONSTRAINT, btree (c)
+    "tt9_c_key3" UNIQUE CONSTRAINT, btree (c)
+Check constraints:
+    "foo" CHECK (c > 3)
+    "tt9_c_check" CHECK (c > 1)
+    "tt9_c_check1" CHECK (c > 2)
+    "tt9_c_key2" CHECK (c > 6)
+
+DROP TABLE tt9;
 -- Check that comments on constraints and indexes are not lost at ALTER TABLE.
 CREATE TABLE comment_test (
   id int,
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index 22cf4ef0a76..b90497804b0 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -1865,6 +1865,24 @@ ALTER TABLE IF EXISTS tt8 SET SCHEMA alter2;
 DROP TABLE alter2.tt8;
 DROP SCHEMA alter2;
 
+--
+-- Check conflicts between index and CHECK constraint names
+--
+CREATE TABLE tt9(c integer);
+ALTER TABLE tt9 ADD CHECK(c > 1);
+ALTER TABLE tt9 ADD CHECK(c > 2);  -- picks nonconflicting name
+ALTER TABLE tt9 ADD CONSTRAINT foo CHECK(c > 3);
+ALTER TABLE tt9 ADD CONSTRAINT foo CHECK(c > 4);  -- fail, dup name
+ALTER TABLE tt9 ADD UNIQUE(c);
+ALTER TABLE tt9 ADD UNIQUE(c);  -- picks nonconflicting name
+ALTER TABLE tt9 ADD CONSTRAINT tt9_c_key UNIQUE(c);  -- fail, dup name
+ALTER TABLE tt9 ADD CONSTRAINT foo UNIQUE(c);  -- fail, dup name
+ALTER TABLE tt9 ADD CONSTRAINT tt9_c_key CHECK(c > 5);  -- fail, dup name
+ALTER TABLE tt9 ADD CONSTRAINT tt9_c_key2 CHECK(c > 6);
+ALTER TABLE tt9 ADD UNIQUE(c);  -- picks nonconflicting name
+\d tt9
+DROP TABLE tt9;
+
 
 -- Check that comments on constraints and indexes are not lost at ALTER TABLE.
 CREATE TABLE comment_test (