Only at the time of reading. If you are transforming some data using a group by , something gênero.Depois the application, you can usually affect when climbing the QVD data to memory and contain foreign key, Cartesian product etc.
To clarify, it is the data loaded into the application (QVW), the number of users accessing that application and the cached calculations that consume RAM, not the QVD files.
QVD- is a format QlikView data, storing data in a special way. So, after storing , qvd-files can be loaded into QlikView applications.
If you do not put logic conditions (except exist) and load a table with no processing from qvd file - loading into memory is very fast. (Optimized load)