There has been a lot of discussion within our group of QV developers and server admins about using QVD's. All of us are new to QlikView and learning together.
In reading information about QVD's, I love the thought of 10-100 times faster.
We are all a little confused on the details of this though.
How is this faster than loading directly from the datasource when one app is created just to load data into the qvd and then the data is loaded again into the app being used?
Does the QVD allow the application being used by end users to read data out of memory faster thus providing a better end user experience?
or...
Does the QVD only reduce the load on the server?
Does it really reduce load on the server, since there is essentially 2 loads being done, one to populate the QVD and one to populate data in the application used by the end user?
I've done a lot of reading on these, but everything I find seems to make the answers to these questions more confusing.
Hi Greg, The whole idea of using qvd's is to not do a lot of processing twice. In developing an app it will save reload time when you need to make changes to the data model, but the real use usually lies in incremental loads. The first time there will be no gain, you load your data from the source and store it in a qvd, but each following time you load the historic data from the qvd and only the new data from the source.
There is no effect in the resulting qlikview file, so end users will not notice anything.
When the qvw file is reloaded on the server, it will save load time in the above mentioned scenario.
By buffering non-changing or slowly-changing blocks of input data in QVD files, script execution becomes considerably faster for large data sets.
Decreasing load on database servers :
The amount of data fetched from external data sources can also be greatly reduced. This reduces the workload on external databases and network traffic. Furthermore, when several scripts share the same data, it is only necessary to load it once from the source database into a QVD file. Other apps can make use of the same data through this QVD file.
Consolidating data from multiple apps :
With the binary script statement, data can be loaded from a single app into another app, but with QVD files a script can combine data from any number of apps. This makes it possible for apps to consolidate similar data from different business units, for example.
Incremental load :
In many common cases, the QVD functionality can be used for incremental load by only loading new records from a growing database.
