Parameter · Reporting and Logging / What to Log
log_replication_commands
Logs each replication command.
- Default:
off - Type: bool
- Applies: Session (superuser)
In plain words
Standbys, backup tools and logical replication speak a special replication protocol with commands like START_REPLICATION. This setting logs each of those commands, which helps troubleshoot replicas and backups that fail to connect or start.
For DBAs and operators
Real output (18.6) from psql "replication=database" -c IDENTIFY_SYSTEM:
A physical standby connecting logs IDENTIFY_SYSTEM, possibly TIMELINE_HISTORY 2, then START_REPLICATION SLOT "standby1" 0/5000000 TIMELINE 1; pg_basebackup logs BASE_BACKUP ( … ); logical subscribers log CREATE_REPLICATION_SLOT … LOGICAL pgoutput and START_REPLICATION SLOT "sub" LOGICAL 0/0 (proto_version '4', …). With the setting off, the same line is still emitted, but at DEBUG1.
Useful when a standby keeps reconnecting (you will see a repeating sequence) or a slot "already active" error. Superuser-settable; enabling server-wide is cheap since replication connections are few.
How to change it
A superuser (or a role granted SET) can change it per session with SET.