A simple query to know when Postgres’ autovacuum will run

Understanding Postgres’ autovacuum isn’t easy at the beginning: it’s a balance between many settings. To help you, here’s a query to understand them and know which tables are pending for autovacuum and how many dead tuples are required before a table gets vacuumed again.

Note: this query uses settings taken from the config files, if you had set special autovacuum parameters for a single table, the won’t be considered.

SELECT psut.relname,
     to_char(psut.last_vacuum, 'YYYY-MM-DD HH24:MI') as last_vacuum,
     to_char(psut.last_autovacuum, 'YYYY-MM-DD HH24:MI') as last_autovacuum,
     to_char(pg_class.reltuples, '9G999G999G999') AS n_tup,
     to_char(psut.n_dead_tup, '9G999G999G999') AS dead_tup,
     to_char(CAST(current_setting('autovacuum_vacuum_threshold') AS bigint)
         + (CAST(current_setting('autovacuum_vacuum_scale_factor') AS numeric)
            * pg_class.reltuples), '9G999G999G999') AS av_threshold,
     CASE
         WHEN CAST(current_setting('autovacuum_vacuum_threshold') AS bigint)
             + (CAST(current_setting('autovacuum_vacuum_scale_factor') AS numeric)
                * pg_class.reltuples) < psut.n_dead_tup
         THEN '*'
         ELSE ''
     END AS expect_av
 FROM pg_stat_user_tables psut
     JOIN pg_class on psut.relid = pg_class.oid
 ORDER BY 1;

The above query is taken from: http://dba.stackexchange.com/questions/21068/aggressive-autovacuum-on-postgresql