Last tested: May 22, 2018
Persistent Derived Tables are automatically reaped by Looker based on the following rules:
If refresh is tied to a particular time (e.g. set via 'persist for'), then the PDT will be reaped from the database 1 hour after the persistence window expires.
PDTs in Dev-Mode are treated as persist-for 24 hours until they are pushed to Production.
If refresh is tied to a SQL Trigger Value, the PDT will continue to be built as long as it exists in a Production model. They will be reaped 1 hour after the associated Production model is deleted.
Datagroups behave based on the sql_trigger within them.