Persistence should not be added to derived tables that will be what?

The correct answer is that persistence should not be added to derived tables that will be extended. In Looker, when a derived table is defined to be extended, it indicates that the table is intended for further modification or enhancement through additional LookML modeling. When you extend a derived table, you're building upon its structure and logic, potentially adding new fields, filters, or other modifications.

Persisting a derived table through database storage (often done to improve performance) contradicts the purpose of extending it, as it would create a static representation that does not allow for these dynamic changes. By avoiding persistence for extended derived tables, you ensure that developers can freely modify the underlying logic without being constrained by a pre-defined dataset in the database.

Choosing persistence for any table that is likely to be extended would lead to inefficiencies and conflicts, as changes would need to be reprocessed each time modifications are made. Therefore, the use of derived tables meant for extension should be done without the persistent storage, allowing flexibility for ongoing development.