I have made a synchronous replication setup with PG12.
During data loading of 200 gig in master node, I found that there is a lag that it shows on the slave. But, ideally for sync replication, lag should be always 0 right.
Because commit never happens in master before updating the slave. log_delay ------------ 871.549671 (1 row) log_delay ------------ 871.557246 (1 row) log_delay ----------- 0 (1 row) log_delay ----------- 0 (1 row)
Query Used for checking Standby Replication Lag:
psql -d perfbench -p 5432 -c "SELECT CASE WHEN pg_last_wal_receive_lsn() = pg_last_wal_replay_lsn() THEN 0 ELSE EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp()) END AS log_delay;"
