check for txn wraparound (when wraparound vacuum freeze needs to occur)

pg_txn_wraparound.sql

SELECT
       oid::regclass::text AS table,
       age(relfrozenxid) AS xid_age, 
       mxid_age(relminmxid) AS mxid_age, 
       least( 
(SELECT setting::int
            FROM    pg_settings
            WHERE   name = 'autovacuum_freeze_max_age') - age(relfrozenxid), 
(SELECT setting::int
            FROM    pg_settings
            WHERE   name = 'autovacuum_multixact_freeze_max_age') - mxid_age(relminmxid)  
) AS tx_before_wraparound_vacuum,
pg_size_pretty(pg_total_relation_size(oid)) AS size,
pg_stat_get_last_autovacuum_time(oid) AS last_autovacuum
FROM    pg_class
WHERE   relfrozenxid != 0
AND oid > 16384
ORDER BY tx_before_wraparound_vacuum;