ALTER TABLE ... ALTER CONSTRAINT

diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 5437626..27146bb 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -46,6 +46,7 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column_name</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
     ADD <replaceable class="PARAMETER">table_constraint</replaceable> [ NOT VALID ]
     ADD <replaceable class="PARAMETER">table_constraint_using_index</replaceable>
+    ALTER CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
     VALIDATE CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable>
     DROP CONSTRAINT [ IF EXISTS ]  <replaceable class="PARAMETER">constraint_name</replaceable> [ RESTRICT | CASCADE ]
     DISABLE TRIGGER [ <replaceable class="PARAMETER">trigger_name</replaceable> | ALL | USER ]
@@ -317,6 +318,15 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
    </varlistentry>
 
    <varlistentry>
+    <term><literal>ALTER CONSTRAINT</literal></term>
+    <listitem>
+     <para>
+      This form alters the attributes of a constraint that was previously created.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
     <term><literal>VALIDATE CONSTRAINT</literal></term>
     <listitem>
      <para>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 8294b29..c29b16e 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -275,6 +275,8 @@ static void AlterIndexNamespaces(Relation classRel, Relation rel,
 static void AlterSeqNamespaces(Relation classRel, Relation rel,
 				   Oid oldNspOid, Oid newNspOid, ObjectAddresses *objsMoved,
 				   LOCKMODE lockmode);
+static void ATExecAlterConstraint(Relation rel, AlterTableCmd *cmd,
+						 bool recurse, bool recursing, LOCKMODE lockmode);
 static void ATExecValidateConstraint(Relation rel, char *constrName,
 						 bool recurse, bool recursing, LOCKMODE lockmode);
 static int transformColumnNameList(Oid relId, List *colList,
@@ -2886,6 +2888,7 @@ AlterTableGetLockLevel(List *cmds)
 			case AT_SetOptions:
 			case AT_ResetOptions:
 			case AT_SetStorage:
+			case AT_AlterConstraint:
 			case AT_ValidateConstraint:
 				cmd_lockmode = ShareUpdateExclusiveLock;
 				break;
@@ -3124,6 +3127,9 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			ATPrepAddInherit(rel);
 			pass = AT_PASS_MISC;
 			break;
+		case AT_AlterConstraint:		/* ALTER CONSTRAINT */
+			ATSimplePermissions(rel, ATT_TABLE);
+			break;
 		case AT_ValidateConstraint:		/* VALIDATE CONSTRAINT */
 			ATSimplePermissions(rel, ATT_TABLE);
 			/* Recursion occurs during execution phase */
@@ -3302,6 +3308,9 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_AddIndexConstraint:		/* ADD CONSTRAINT USING INDEX */
 			ATExecAddIndexConstraint(tab, rel, (IndexStmt *) cmd->def, lockmode);
 			break;
+		case AT_AlterConstraint:		/* ALTER CONSTRAINT */
+			ATExecAlterConstraint(rel, cmd, false, false, lockmode);
+			break;
 		case AT_ValidateConstraint:		/* VALIDATE CONSTRAINT */
 			ATExecValidateConstraint(rel, cmd->name, false, false, lockmode);
 			break;
@@ -6174,6 +6183,130 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 }
 
 /*
+ * ALTER TABLE ALTER CONSTRAINT
+ *
+ * Update the attributes of a constraint.
+ */
+static void
+ATExecAlterConstraint(Relation rel, AlterTableCmd *cmd,
+						 bool recurse, bool recursing, LOCKMODE lockmode)
+{
+	Relation	conrel;
+	SysScanDesc scan;
+	ScanKeyData key;
+	HeapTuple	contuple;
+	Form_pg_constraint currcon = NULL;
+	Constraint	*cmdcon = NULL;
+	bool		found = false;
+
+	Assert(IsA(cmd->def, Constraint));
+	cmdcon = (Constraint *) cmd->def;
+
+	conrel = heap_open(ConstraintRelationId, RowExclusiveLock);
+
+	/*
+	 * Find and check the target constraint
+	 */
+	ScanKeyInit(&key,
+				Anum_pg_constraint_conrelid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(RelationGetRelid(rel)));
+	scan = systable_beginscan(conrel, ConstraintRelidIndexId,
+							  true, SnapshotNow, 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;
+		}
+	}
+
+	if (!found)
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_OBJECT),
+				 errmsg("constraint \"%s\" of relation \"%s\" does not exist",
+						cmdcon->conname, RelationGetRelationName(rel))));
+
+	if (currcon->contype != CONSTRAINT_FOREIGN)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("constraint \"%s\" of relation \"%s\" is not a foreign key constraint",
+						cmdcon->conname, RelationGetRelationName(rel))));
+
+	if (currcon->condeferrable != cmdcon->deferrable ||
+		currcon->condeferred != cmdcon->initdeferred)
+	{
+		HeapTuple	copyTuple;
+		HeapTuple	tgtuple;
+		Form_pg_constraint copy_con;
+		Form_pg_trigger copy_tg;
+		ScanKeyData tgkey;
+		SysScanDesc tgscan;
+		Relation	tgrel;
+
+		/*
+		 * Now update the catalog, while we have the door open.
+		 */
+		copyTuple = heap_copytuple(contuple);
+		copy_con = (Form_pg_constraint) GETSTRUCT(copyTuple);
+		copy_con->condeferrable = cmdcon->deferrable;
+		copy_con->condeferred = cmdcon->initdeferred;
+		simple_heap_update(conrel, &copyTuple->t_self, copyTuple);
+		CatalogUpdateIndexes(conrel, copyTuple);
+
+		InvokeObjectPostAlterHook(ConstraintRelationId,
+								  HeapTupleGetOid(contuple), 0);
+
+		heap_freetuple(copyTuple);
+
+		/*
+		 * Now we need to update the multiple entries in pg_trigger
+		 * that implement the constraint.
+		 */
+		tgrel = heap_open(TriggerRelationId, RowExclusiveLock);
+
+		ScanKeyInit(&tgkey,
+					Anum_pg_trigger_tgconstraint,
+					BTEqualStrategyNumber, F_OIDEQ,
+					ObjectIdGetDatum(HeapTupleGetOid(contuple)));
+
+		tgscan = systable_beginscan(tgrel, TriggerConstraintIndexId, true,
+									SnapshotNow, 1, &tgkey);
+
+		while (HeapTupleIsValid(tgtuple = systable_getnext(tgscan)))
+		{
+			copyTuple = heap_copytuple(tgtuple);
+			copy_tg = (Form_pg_trigger) GETSTRUCT(copyTuple);
+			copy_tg->tgdeferrable = cmdcon->deferrable;
+			copy_tg->tginitdeferred = cmdcon->initdeferred;
+			simple_heap_update(tgrel, &copyTuple->t_self, copyTuple);
+			CatalogUpdateIndexes(tgrel, copyTuple);
+
+			InvokeObjectPostAlterHook(TriggerRelationId,
+										HeapTupleGetOid(tgtuple), 0);
+
+			heap_freetuple(copyTuple);
+		}
+
+		systable_endscan(tgscan);
+
+		heap_close(tgrel, RowExclusiveLock);
+
+		/*
+		 * Invalidate relcache so that others see the new attributes.
+		 */
+		CacheInvalidateRelcache(rel);
+	}
+
+	systable_endscan(scan);
+
+	heap_close(conrel, RowExclusiveLock);
+}
+
+/*
  * ALTER TABLE VALIDATE CONSTRAINT
  *
  * XXX The reason we handle recursion here rather than at Phase 1 is because
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 5094226..5e7db52 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -1941,6 +1941,21 @@ alter_table_cmd:
 					n->def = $2;
 					$$ = (Node *)n;
 				}
+			/* ALTER TABLE <name> ALTER CONSTRAINT ... */
+			| ALTER CONSTRAINT name ConstraintAttributeSpec
+				{
+					AlterTableCmd *n = makeNode(AlterTableCmd);
+					Constraint *c = makeNode(Constraint);
+					n->subtype = AT_AlterConstraint;
+					n->def = (Node *) c;
+					c->contype = CONSTR_FOREIGN; /* others not supported, yet */
+					c->conname = $3;
+					processCASbits($4, @4, "ALTER CONSTRAINT statement",
+									&c->deferrable,
+									&c->initdeferred,
+									NULL, NULL, yyscanner);
+					$$ = (Node *)n;
+				}
 			/* ALTER TABLE <name> VALIDATE CONSTRAINT ... */
 			| VALIDATE CONSTRAINT name
 				{
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 6723647..9453e1d 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1209,6 +1209,7 @@ typedef enum AlterTableType
 	AT_AddConstraint,			/* add constraint */
 	AT_AddConstraintRecurse,	/* internal to commands/tablecmds.c */
 	AT_ReAddConstraint,			/* internal to commands/tablecmds.c */
+	AT_AlterConstraint,			/* alter constraint */
 	AT_ValidateConstraint,		/* validate constraint */
 	AT_ValidateConstraintRecurse,		/* internal to commands/tablecmds.c */
 	AT_ProcessedConstraint,		/* pre-processed add constraint (local in
diff --git a/src/test/regress/expected/foreign_key.out b/src/test/regress/expected/foreign_key.out
index 04668a8..0299bfe 100644
--- a/src/test/regress/expected/foreign_key.out
+++ b/src/test/regress/expected/foreign_key.out
@@ -1132,6 +1132,15 @@ CREATE TEMP TABLE fktable (
     id int primary key,
     fk int references pktable deferrable initially deferred
 );
+-- check ALTER CONSTRAINT
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey NOT DEFERRABLE;
+-- illegal option
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey NOT DEFERRABLE INITIALLY DEFERRED;
+ERROR:  constraint declared INITIALLY DEFERRED must be DEFERRABLE
+LINE 1: ...e ALTER CONSTRAINT fktable_fk_fkey NOT DEFERRABLE INITIALLY ...
+                                                             ^
+-- reset
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey DEFERRABLE INITIALLY DEFERRED;
 INSERT INTO pktable VALUES (5, 10);
 BEGIN;
 -- doesn't match PK, but no error yet
@@ -1142,6 +1151,16 @@ UPDATE fktable SET id = id + 1;
 COMMIT;
 ERROR:  insert or update on table "fktable" violates foreign key constraint "fktable_fk_fkey"
 DETAIL:  Key (fk)=(20) is not present in table "pktable".
+-- change the constraint definition and retest
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey DEFERRABLE INITIALLY IMMEDIATE;
+BEGIN;
+-- doesn't match PK, should throw error now
+INSERT INTO fktable VALUES (0, 20);
+ERROR:  insert or update on table "fktable" violates foreign key constraint "fktable_fk_fkey"
+DETAIL:  Key (fk)=(20) is not present in table "pktable".
+COMMIT;
+-- reset
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey DEFERRABLE INITIALLY DEFERRED;
 -- check same case when insert is in a different subtransaction than update
 BEGIN;
 -- doesn't match PK, but no error yet
diff --git a/src/test/regress/sql/foreign_key.sql b/src/test/regress/sql/foreign_key.sql
index 377b36c..531c881 100644
--- a/src/test/regress/sql/foreign_key.sql
+++ b/src/test/regress/sql/foreign_key.sql
@@ -818,6 +818,13 @@ CREATE TEMP TABLE fktable (
     fk int references pktable deferrable initially deferred
 );
 
+-- check ALTER CONSTRAINT
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey NOT DEFERRABLE;
+-- illegal option
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey NOT DEFERRABLE INITIALLY DEFERRED;
+-- reset
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey DEFERRABLE INITIALLY DEFERRED;
+
 INSERT INTO pktable VALUES (5, 10);
 
 BEGIN;
@@ -831,6 +838,19 @@ UPDATE fktable SET id = id + 1;
 -- should catch error from initial INSERT
 COMMIT;
 
+-- change the constraint definition and retest
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey DEFERRABLE INITIALLY IMMEDIATE;
+
+BEGIN;
+
+-- doesn't match PK, should throw error now
+INSERT INTO fktable VALUES (0, 20);
+
+COMMIT;
+
+-- reset
+ALTER TABLE fktable ALTER CONSTRAINT fktable_fk_fkey DEFERRABLE INITIALLY DEFERRED;
+
 -- check same case when insert is in a different subtransaction than update
 
 BEGIN;