Add pg_advance_vacuum_cleanup_age().

This function specifies the number of transactions by
which VACUUM and HOT updates will advance cleanup
of dead row versions.

README.md

@@ -167,6 +167,29 @@ This function returns a record, shown in the table below.
 This function is restricted to superusers by default,
 but other users can be granted EXECUTE to run the function.
 
+### integer pg_advance_vacuum_cleanup_age(integer)
+Specify the number of transactions by which VACUUM and HOT updates
+will advance cleanup of dead row versions.
+[vacuum_defer_cleanup_age](https://www.postgresql.org/docs/devel/static/runtime-config-replication.html#GUC-VACUUM-DEFER-CLEANUP-AGE)
+in the session calling this function is set to
+the negative value of that specified number. If the argument is omitted
+or NULL is specified, vacuum_defer_cleanup_age is reset to
+its original setting value specified in the configuration file.
+This function returns the vacuum cleanup age.
+
+By advancing the cleanup age, VACUUM and HOT updates can clean up
+even dead tuples that were produced since oldest transaction had started.
+So this function is helpful to prevent the database from bloating due to
+unremovable dead tuples while long transaction is running.
+
+Note that this is extremely dangerous function and can easily corrupt
+the database. Any important data may disappear and data consistency
+may be lost completely.
+This function must not be used for a purpose other than debug.
+
+This function is restricted to superusers by default,
+but other users can be granted EXECUTE to run the function.
+
 ### void pg_checkpoint(fast bool, wait bool, force bool)
 Perform a checkpoint.
 If fast is true (default), a checkpoint will finish as soon as possible.

pg_cheat_funcs--1.0.sql

@@ -122,6 +122,12 @@ AS 'MODULE_PATHNAME'
 LANGUAGE C STRICT VOLATILE;
 REVOKE ALL ON FUNCTION pg_oid_assignment() FROM PUBLIC;
 
+CREATE FUNCTION pg_advance_vacuum_cleanup_age(integer DEFAULT NULL)
+RETURNS integer
+AS 'MODULE_PATHNAME'
+LANGUAGE C CALLED ON NULL INPUT VOLATILE;
+REVOKE ALL ON FUNCTION pg_advance_vacuum_cleanup_age(integer) FROM PUBLIC;
+
 CREATE FUNCTION pg_checkpoint(bool DEFAULT true, bool DEFAULT true,
     bool DEFAULT true)
 RETURNS void

pg_cheat_funcs.c

@@ -97,6 +97,7 @@ PG_FUNCTION_INFO_V1(pg_set_next_xid);
 PG_FUNCTION_INFO_V1(pg_xid_assignment);
 PG_FUNCTION_INFO_V1(pg_set_next_oid);
 PG_FUNCTION_INFO_V1(pg_oid_assignment);
+PG_FUNCTION_INFO_V1(pg_advance_vacuum_cleanup_age);
 PG_FUNCTION_INFO_V1(pg_checkpoint);
 PG_FUNCTION_INFO_V1(pg_promote);
 PG_FUNCTION_INFO_V1(pg_recovery_settings);
@@ -124,6 +125,7 @@ Datum pg_set_next_xid(PG_FUNCTION_ARGS);
 Datum pg_xid_assignment(PG_FUNCTION_ARGS);
 Datum pg_set_next_oid(PG_FUNCTION_ARGS);
 Datum pg_oid_assignment(PG_FUNCTION_ARGS);
+Datum pg_advance_vacuum_cleanup_age(PG_FUNCTION_ARGS);
 Datum pg_checkpoint(PG_FUNCTION_ARGS);
 Datum pg_promote(PG_FUNCTION_ARGS);
 Datum pg_recovery_settings(PG_FUNCTION_ARGS);
@@ -843,6 +845,35 @@ pg_oid_assignment(PG_FUNCTION_ARGS)
 						heap_form_tuple(tupdesc, values, nulls)));
 }
 
+/*
+ * Specify the number of transactions by which VACUUM and HOT updates
+ * will advance cleanup of dead row versions.
+ */
+Datum
+pg_advance_vacuum_cleanup_age(PG_FUNCTION_ARGS)
+{
+	static bool	advanced = false;
+	static int		save_cleanup_age = 0;
+	static int		orig_cleanup_age = 0;
+
+	if (!advanced || vacuum_defer_cleanup_age != save_cleanup_age)
+		orig_cleanup_age = vacuum_defer_cleanup_age;
+
+	if (PG_ARGISNULL(0))
+	{
+		vacuum_defer_cleanup_age = orig_cleanup_age;
+		advanced = false;
+	}
+	else
+	{
+		vacuum_defer_cleanup_age = - PG_GETARG_INT32(0);
+		advanced = true;
+	}
+	save_cleanup_age = vacuum_defer_cleanup_age;
+
+	PG_RETURN_INT32(- vacuum_defer_cleanup_age);
+}
+
 /*
  * Perform a checkpoint.
  */

sql/pg_cheat_funcs.sql

@@ -10,6 +10,21 @@ SELECT pg_eucjp('xa4', 'xa2');
 
 SELECT pg_set_next_xid(next_xid) = next_xid FROM pg_xid_assignment();
 
+DO $$
+DECLARE
+  orig_cleanup_age text := current_setting('vacuum_defer_cleanup_age');
+BEGIN
+  PERFORM pg_advance_vacuum_cleanup_age(99);
+  IF current_setting('vacuum_defer_cleanup_age') <> '-99' THEN
+    RAISE WARNING 'could not advance vacuum cleanup age properly.';
+  END IF;
+  PERFORM pg_advance_vacuum_cleanup_age();
+  IF current_setting('vacuum_defer_cleanup_age') <> orig_cleanup_age THEN
+    RAISE NOTICE 'could not reset vacuum cleanup age properly.';
+  END IF;
+END;
+$$ LANGUAGE plpgsql;
+
 SELECT to_octal(num) FROM generate_series(1, 10) num;
 SELECT to_octal(2147483647::integer);
 SELECT to_octal(9223372036854775807::bigint);