Hi Steve,
the query builder function query.functions.custom(name, args...) is supported also in 8.3.3 and previous versions. This function allow to run any custom function available in the DB within the Query Builder.
Not sure why you get the error, maybe a typo; have you tried to test the function directly within your database (using any database tool) ?
About in-memory datasources, that is a very interesting question, honestly i don't know if is possible at all.. i would try at first to test if i can run any of the built in function of HyperSQL as a custom function (
http://hsqldb.org/doc/guide/builtinfunctions-chapt.html)
If the built in functions can't be used to obtain the datepart, i don't know if is possible to apply the same solution as for SQL Server; since the in-memory DB is created dynamically using UUID as Table and Column names, i am not sure if you can alter at all the DB using rawSQL.. there is surely a challenge to map the table/column names to the proper UUIDs.
Maybe one of our R&D experts can tell something more about it.
Regards,
Paolo