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Find Out Postgresql Replication Delay/Lag using Functions

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By comparing Below two values you could see if the servers are in sync.The problem yet again is that if streaming replication fails, both of these functions will continue to return same values and you could still end up thinking the replication is working. But also you need to query both the master and slave to be able to monitor this, which is not that easy on monitoring systems, and you still don't have the information about the actual lag in seconds, so you would still need to run the first query.
Master:

SELECT pg_current_xlog_location();
Slave:

SELECT pg_last_xlog_receive_location();





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