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Checking PostgreSQL Replication Delay

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You can get the delay in bytes from the master side quite easily using pg_xlog_location_diff to compare the master's pg_current_xlog_insert_location with the replay_location for that backend's pg_stat_replication entry.

postgres=# SELECT
 pg_last_xlog_receive_location() receive,
 pg_last_xlog_replay_location() replay,
 (
  extract(epoch FROM now()) -
  extract(epoch FROM pg_last_xact_replay_timestamp())
 )::int lag;

 receive   |   replay   |  lag
------------+------------+-------
1/AB861728 | 1/AB861728 | 2027

the lag is only only important when receive is different than replay. execute the query on the slave

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