Parameter · Developer Options
allow_system_table_mods
Allows modifications of the structure of system tables.
- Default:
off - Type: bool
- Applies: Session (superuser)
In plain words
The system catalogs (pg_class, pg_attribute, …) describe every object in the database. PostgreSQL refuses to let anyone, even a superuser, change their structure, because a mistake can make the whole database unreadable.
This switch removes that safety catch. Ordinary UPDATEs on catalogs by superusers are already allowed without it.
For DBAs and operators
Captured on 18.6:
ALTER TABLE pg_class ADD COLUMN foo int;ERROR: permission denied: "pg_class" is a system catalogWith SET allow_system_table_mods = on such DDL is accepted (for example CREATE INDEX on pg_class succeeded in a rolled-back test). It covers structural changes and a few other risky operations, such as creating objects in pg_catalog or changing catalog storage options.
Legitimate uses are rare: PostgreSQL developers, upgrade tooling, or a carefully scripted repair given by someone who knows the catalog code. Hand-edited catalogs are a frequent root cause of "unexplainable" errors and of pg_upgrade/pg_dump failures; when you see strange catalog state in a support case, ask whether this setting (or direct catalog UPDATEs) was ever used.
Danger. Mistakes can corrupt the catalogs beyond repair and lose the whole database. Take a file-level copy of the data directory (or of the affected files) before touching it, and turn it off again as soon as you are done.
How to change it
A superuser (or a role granted SET) can change it per session with SET.