High number of live/dead tuples in postgresql/ Vacuum not working

Sahil Aggarwal :

There is a table , which has 200 rows . But number of live tuples showing there is more than that (around 60K) .

select count(*) from subscriber_offset_manager;
 count 
-------
   200
(1 row)


 SELECT schemaname,relname,n_live_tup,n_dead_tup FROM pg_stat_user_tables  where relname='subscriber_offset_manager' ORDER BY n_dead_tup
;
 schemaname |          relname          | n_live_tup | n_dead_tup 
------------+---------------------------+------------+------------
 public     | subscriber_offset_manager |      61453 |          5
(1 row)

But as seen from pg_stat_activity and pg_locks , we are not able to track any open connection .

SELECT query, state,locktype,mode
FROM pg_locks
JOIN pg_stat_activity
  USING (pid)
WHERE relation::regclass = 'subscriber_offset_manager'::regclass
  ;
 query | state | locktype | mode 
-------+-------+----------+------
(0 rows)

I also tried full vacuum on this table , Below are results :

• All the times no rows are removed
• some times all the live tuples become dead tuples .

Here is output .

vacuum FULL VERBOSE ANALYZE subscriber_offset_manager;
INFO:  vacuuming "public.subscriber_offset_manager"
INFO:  "subscriber_offset_manager": found 0 removable, 67920 nonremovable row versions in 714 pages
DETAIL:  67720 dead row versions cannot be removed yet.
CPU 0.01s/0.06u sec elapsed 0.13 sec.
INFO:  analyzing "public.subscriber_offset_manager"
INFO:  "subscriber_offset_manager": scanned 710 of 710 pages, containing 200 live rows and 67720 dead rows; 200 rows in sample, 200 estimated total rows
VACUUM

 SELECT schemaname,relname,n_live_tup,n_dead_tup FROM pg_stat_user_tables  where relname='subscriber_offset_manager' ORDER BY n_dead_tup
;
 schemaname |          relname          | n_live_tup | n_dead_tup 
------------+---------------------------+------------+------------
 public     | subscriber_offset_manager |        200 |      67749

and after 10 sec

SELECT schemaname,relname,n_live_tup,n_dead_tup FROM pg_stat_user_tables  where relname='subscriber_offset_manager' ORDER BY n_dead_tup
;
 schemaname |          relname          | n_live_tup | n_dead_tup 
------------+---------------------------+------------+------------
 public     | subscriber_offset_manager |      68325 |        132

How Our App query to this table .

• Our application generally select some rows and based on some business calculation, update the row .

  select query -- select based on some id

  select * from subscriber_offset_manager where shard_id=1 ;

  update query -- update some other column for this selected shard id

• around 20 threads do this in parallel and One thread works on only one row .

• app is writen in java and we are using hibernate to do db operations .
• Postgresql version is 9.3.24

One more interesting observation : - when i stop my java app and then do full vacuum , it works fine (number of rows and live tuples become equal). So there is something wrong if we select and update continuously from java app . –

Problem/Issue

These live tuples some times go to dead tuples and after some times again comes to live .

Due to above behaviour select from the table taking time and increasing load on server as lots of live/deadtuples are there ..

Laurenz Albe :

I know three things that keep VACUUM from doing its job:

• Long running transactions.

• Prepared transactions that did not get committed.

• Stale replication slots.

See my blog post for details.