There is some stuff that happens in separate custom functions, but all the functions consist of supported operations.

The aggregation is .groupBy("Name").count() before anything happens as a way to distinct the data. (because 'distinct' is not supported but 'group by' is).
Afterwards it's a bit of processing and then finally a join on some fields extracted from the 'Name'.

So Materialized Views don't support joins as a final step?