Determining replication lag

I found the following SQL in stackoverflow:

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

I get different result in primary and slave. On primary the col lag always
shows some number, usually more than 3000, but on slave it is always zero.
Why the discrepancy ?





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