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I am new to QlikView have difficulty to understand for QVD.
The QVD file format is just a binary, compact format to store one single table in a file. When you load it - and you don't make any changes - it loads optimized, i.e. extremely fast. But if you make any transformations, or unpacks record-wise data in the Load statement, it loads slower; it loads non-optimized. That's all.
Apart from this, you can use a QVD file in the same way as a CSV: concatenate them, join them, etc.
When you are loading data from QVD without any changes then it is called Optimized load, i.e., it loads very faster, if you doing any transformation, calculations, filtering using where condition, sorting then it is called Non Optimized load.
Hope it helps you.
"Optimized" has nothing to do with how the QVD is created. There is no such thing as an "optimized file" or a "non-optimized file". The only thing that matters, is how the QVD file is loaded - whether the Load statement contains expressions that forces QlikView to unpack the QVD during the Load sequence.