QlikView or Qlik Sense reads from a QVD in fast mode by default.
You can improve the time to reload by employing an optimized QVD load.
In nutshell, optimized load allows QlikView or Qlik Sense to read QVD in super-fast mode or faster than regular QVD load. This translates into major saving in reload time when source QVD is large.
The rules for an optimized QVD load are simple.
1. You can’t transform any field while reading QVD except aliasing a field.
2. You can use EXISTS function only if you use single parameter.
For example, EXISTS function can be like WHERE EXISTS(Pat_ID, Patient_ID) if you are trying to compare previously loaded Pat_IDs against Patient_ID field from the current load. This will not be permitted during an optimized load.
However, same EXISTS function with single field is allowed during the optimized load.
For example, if you are comparing previously load Patinet_ID field from another table against Patient_ID loaded from the current LOAD statement, you can write a short form WHERE EXISTS(Patient_ID) and this will be allowed during the optimized load
I found additional quirk to these rules while troubleshooting a seemingly optimized load that was not an optimized load.
3. You can’t alias a field that has been used in the EXISTS function
Let’s look at an example:
This is an optimized load:
However, this is NOT an optimized load:
