> [!NOTE] ![[assets/wl_lwlock_relationmapping.png|256]] > > | | | > | :--- | :--- | > | **Wait Category** | LWLock | > | **Mechanism** | `RelationMappingLock` | > | **Operations** | [[Operations/Table/SeqScan]], [[Operations/Tuple/ModifyTable]] | > | **Resources** | [[Resources/RAM|Memory]], [[Resources/CPU|CPU]] | > > Contention on the **Physical Identity Layer**. While most relations store their file identifiers in the `pg_class` catalog, core system catalogs (and objects being currently rewritten) rely on a dedicated mapping file to ensure that the engine can always find its "bootstrap" data even during recovery or after a crash.