Optimized loads are possible if no transformations (calculations) are made during the load, and no (almost) filtering is done in a WHERE clause.
The only acceptable filter is using function Exists:
WHERE Exists(Field)
So for example this load can't be optimized.
Load * From ... WHERE Country = 'US
but this load can:
Load * From ... WHERE EXISTS(Country)
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On the other hand, the only time you want the load not to be optimized, is if you load a Mapping table from a QVD file. In this case, Optimized load doesn't quite work, so you want to make the load non-optimized deliberately.
QVD : Stands for Qlikview Data (One of the optimize layer of data which has been fetched from the database, qvd is a patent technology of qliktech and qlikview applications reads data 100 time faster as compared to any other sources because data get stores in optimized format. Only Qlikvew application can read the data from QVD. Its always better to fetch the data from from database first and then load the data into your application)
