It appears that the Designer Preview always imports data. Please give a choice between caching and direct queries.
I have the same problem with ORACLE connections. my 3 mil query need 3 hours to update, and its bothersome when you just change a datype or add a calculated column, which doesn't need reread the source data. There should be a clear separation between the data that needs to be fetched and the mutations that should be done to that data. An alternative could be to be able to manipulate the query in the powerbi com. Loading in powerBI .com this would enable met to develop with subsets and load the full data after I published it.
