Luca Ferrari wrote: > now this should be trivial, but I cannot udnerstand what is the > purpose of session_replication_role > or better, when I should use it in a way different from 'origin'.
It is used to enable or disable triggers.
By default, tables are created with all triggers enabled, which means that they fire with the default setting "session_replication_role = origin".
You can change "session_replication_role" to "replica" to disable the firing of triggers (unless they are set ENABLE REPLICA or ENABLE ALWAYS). This is done by the logical replication apply worker, but you can also use it to bypass triggers, e.g. to speed up operation, if you know what you are doing.
What is confusing is that there are three settings for "session_replication_role", but the two settings "local" and "origin" have the same meaning. Maybe that was meant to change at some point, but I see no explanation in the original discussion.
All of the above does also apply to referential integrity triggers. That means that under session_replication_role='replica' you replication system can replicate things out of order with respect to foreign keys. It also means that if you don't replicate the primary key table you can get the target database inconsistent.
The setting of 'local' has indeed the same meaning for everything in stock PostgreSQL. The Slony log and deny-access triggers react to it by suppressing their actions. An application working under 'local' can modify the origin without the changes being replicated and modify the replica without the deny-access trigger aborting the transaction. The Slony engine uses that mode when running SQL scripts through the EXECUTE DDL feature. That way you can perform bulk operations like pruning without the individual row changes being replicated.
The setting of 'replica' is very important if you have triggers that for example do auditing or stuff like stamping created and last update timestamps or session users. You certainly don't want to overwrite the real last update timestamp or session user with the replication engine user and time.
