obviously the connector cannot handle aggregations. Direct Discovery is a feature which uses SQL aggregation queries only which consists of a GROUP BY clause and aggregation functions (count/min/max/sum/avg etc. for charts).
Usually this functionality is only provided by a real database. I wonder if you're going to implement this in a connector..
The reason I would like to use my own custom connector is that I don't the Qlikview to be soliciting my database whenever the user selection changes.
Also, because of the size of the data I need to pull, I would like to be able to stream it during run time so that it is not loaded in-memory all at once.
To address this problem I was thinking about creating my own datastore and use a custom connector to query it and stream it back to Qlikview.
Direct Discovery is not currently supported within QVX, only for the ODBC based connections. As for Qlikview soliciting the source db, there are some parameters you can set to throttle and control the load that is placed and also the duration for refresh of a Qlikview side cache that will hold previous results.