Most of the times, I prefer storing the data into QVD's as a first step. Basically this enables me;
1. Join data from various resources in a single file,
2. Add new calculated fields at once and store them in the QVD so that they will be available for use at any time,
3. Simplify the load script of the end-user QVW and thus substantially reduce the reload time of the end-user
Hope this helps.
QVD's are internal storage in qlikview .these are compressed files.generally we get the tables from different sources we store them into QVD's.Loading data from QVD's are 10 to 100 times faster than fetching the data from normal tables.
1) Reduce the network traffic because we store tables internally as QVD
2) No burden on DB.
A QVD stores ONE table, quite the same way as QV stores this table in memory
it is why the load is very fast : everything is organized in the QVD as it should be in QV
Because it is so fast, we store part of the data of an application in the QVD, and part of the data must be fetched in the source files or databases.