From @berylc - possibly due to empty partitions? This was after 2 import_table's and a join: Java stack trace: java.lang.UnsupportedOperationException: empty.min at scala.collection.TraversableOnce ...

This corresponds to the following protocol messages sent from the frontend to the backend: Parse (P) with an empty string for the statement name and the query Bind (B) with C_1 for the portal, an ...

The Collections class’s methods for obtaining empty collections are preferable to use of Collections ‘s similarly named fields for that same purpose because of the type safety the methods provide.