Fix DROP ACCESS METHOD IF EXISTS.

The IF EXISTS option was documented, and implemented in the grammar, but
it didn't actually work for lack of support in does_not_exist_skipping().
Per bug #14160.

Report and patch by Kouhei Sutou

Report: <20160527070433.19424.81712@wrigleys.postgresql.org>

src/backend/commands/dropcmds.c

@@ -262,6 +262,10 @@ does_not_exist_skipping(ObjectType objtype, List *objname, List *objargs)
 
 	switch (objtype)
 	{
+		case OBJECT_ACCESS_METHOD:
+			msg = gettext_noop("access method \"%s\" does not exist, skipping");
+			name = NameListToString(objname);
+			break;
 		case OBJECT_TYPE:
 		case OBJECT_DOMAIN:
 			{
@@ -438,7 +442,7 @@ does_not_exist_skipping(ObjectType objtype, List *objname, List *objargs)
 			}
 			break;
 		default:
-			elog(ERROR, "unexpected object type (%d)", (int) objtype);
+			elog(ERROR, "unrecognized object type: %d", (int) objtype);
 			break;
 	}
 

src/test/regress/expected/drop_if_exists.out

@@ -227,6 +227,11 @@ DROP OPERATOR FAMILY test_operator_family USING no_such_am;
 ERROR:  access method "no_such_am" does not exist
 DROP OPERATOR FAMILY IF EXISTS test_operator_family USING no_such_am;
 ERROR:  access method "no_such_am" does not exist
+-- access method
+DROP ACCESS METHOD no_such_am;
+ERROR:  access method "no_such_am" does not exist
+DROP ACCESS METHOD IF EXISTS no_such_am;
+NOTICE:  access method "no_such_am" does not exist, skipping
 -- drop the table
 DROP TABLE IF EXISTS test_exists;
 DROP TABLE test_exists;

src/test/regress/sql/drop_if_exists.sql

@@ -227,6 +227,10 @@ DROP OPERATOR FAMILY IF EXISTS test_operator_family USING btree;
 DROP OPERATOR FAMILY test_operator_family USING no_such_am;
 DROP OPERATOR FAMILY IF EXISTS test_operator_family USING no_such_am;
 
+-- access method
+DROP ACCESS METHOD no_such_am;
+DROP ACCESS METHOD IF EXISTS no_such_am;
+
 -- drop the table
 
 DROP TABLE IF EXISTS test_exists;