fairly serious bug with pg_autovacuum in pg7.4

[Brian Hirt]

Hello,

I've run across a pretty serious problem with pg_autovacuum.
pg_autovacuum looses track of any table that's ever been truncated
(possibly other situations too). When i truncate a table it gets a
new relfilenode in pg_class. This is a problem because pg_autovacuum
assumes pg_class.relfilenode will join to pg_stats_all_tables.relid.
pg_stats_all_tables.relid is actallly the oid from pg_class, not the
relfilenode. These two values start out equal so pg_autovacuum works
initially, but it fails later on because of this incorrect assumption.

here is one query pg_autovacuum uses (from pg_autovacuum.h) to get
tables that breaks.

select
a.relfilenode,a.relname,a.relnamespace,a.relpages, a.relisshared,a.reltup
les,b.schemaname,b
..n_tup_ins,b.n_tup_upd,b.n_tup_del from pg_class a, pg_stat_all_tables
b where a.relfilenode=b.relid and a.relkind = 'r'
here's a little test case you can use to see what happens:

basement=# create table test_table ( id int4 );
CREATE TABLE
basement=# select relname, relfilenode from pg_class where relkind =
'r' and relname = 'test_table';
relname | relfilenode
------------+-------------
test_table | 28814151
(1 row)

basement=# select relid,relname from pg_stat_all_tables where relname =
'test_table';
relid | relname
----------+------------
28814151 | test_table
(1 row)

basement=# select
a.relfilenode,a.relname,a.relnamespace,a.relpages, a.relisshared,a.reltup
les,b.schemaname,
basement-# b.n_tup_ins,b.n_tup_upd,b.n_tup_del from pg_class a,
pg_stat_all_tables b
basement-# where a.relfilenode=b.relid and a.relkind = 'r' and
a.relname = 'test_table';
relfilenode | relname | relnamespace | relpages | relisshared |
reltuples | schemaname | n_tup_ins | n_tup_upd | n_tup_del
-------------+------------+--------------+----------+-------------
+-----------+------------+-----------+-----------+-----------
28814151 | test_table | 2200 | 10 | f |
1000 | public | 0 | 0 | 0
(1 row)

basement=#
basement=# truncate table test_table;
TRUNCATE TABLE
basement=# select relname, relfilenode from pg_class where relkind =
'r' and relname = 'test_table';
relname | relfilenode
------------+-------------
test_table | 28814153
(1 row)

basement=# select relid,relname from pg_stat_all_tables where relname =
'test_table';
relid | relname
----------+------------
28814151 | test_table
(1 row)

basement=# select
a.relfilenode,a.relname,a.relnamespace,a.relpages, a.relisshared,a.reltup
les,b.schemaname,
basement-# b.n_tup_ins,b.n_tup_upd,b.n_tup_del from pg_class a,
pg_stat_all_tables b
basement-# where a.relfilenode=b.relid and a.relkind = 'r' and
a.relname = 'test_table';
relfilenode | relname | relnamespace | relpages | relisshared |
reltuples | schemaname | n_tup_ins | n_tup_upd | n_tup_del
-------------+---------+--------------+----------+-------------
+-----------+------------+-----------+-----------+-----------
(0 rows)

basement=# drop table test_table;
DROP TABLE
basement=#
PS: i'm running pg-7.4 and pg_autovacuum from contrib.
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