In case of large SCEC MySQL databases, the expunge process ( deleting already populated data in the mysql db for a workflow ) takes a long time, resulting in a spike in the memory usage.

This is because of the way expunge has been implemented in the netlogger SQL Alchemy code.
It triggers individual select statements , the results of which are then invidually deleted.

The Expunge code already does things specific to a database backend
1) for sqlite it just overwrites

Hence, for mysql it is possible to rely on database triggers in the schema definition to do cascaded deletes