When does old PDT get deleted?

[MattChan]

I am seeing the error “relation ‘looker_scratch_XXXXX’ does not exist” quite often. This usually resolves itself after a while or if I force “Rebuild Derived Table and Run” manually.
I suspect that it’s because the the PDT expired/got triggered and this error happens between the time when the old PDT gets deleted and the new PDT is still being built.
Does old PDT get deleted as soon as the SQL_trigger activates, or when someone try to rebuild manually? Or does it wait until there is a new PDT?

In my case, I am using datagroup, which supposed to rebuild the PDT based on the SQL_trigger (which get queried every 5 min).