When using streaming replication in PostgreSQL 9, it's important to know what the latency is between the master and slaves, especially when deploying on cloud based instances. Ideally, we'd like to know by how many bytes the WAL logs are lagging. PostgreSQL offers a neat way to check just that between a given slave and its master via the pg_current_xlog_location() and pg_last_xlog_replay_location() functions. However, the output seems cryptic.