Definition And Advantages of QVDs in QlikView
QlikView Data Files
QlikView QVDs play a crucial part in the majority of QlikView implementations.
What Is A QVD?
Basically, a QVD is a flat data file with the extension .qvd. It can store a single table of data and is typically created in the load script of a QVW file. The structure of the file is essentially an XML format, with various bits of header information stored at the top of the file and the rest of the data beneath. One of the beauties of a QVD file is that it is compressed on creation using the same algorithms as QlikView uses to store data in memory – so the files can be incredibly small for the amount of data they contain. Loading from QVD back into memory is blindingly fast as the format of the file mirrors how QlikView addresses data in RAM.
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This particular field describes the creation and use of qlikview data files (QVDs). QVDs are important in most qlikview application projects as they provide a means of batch extractions from a data source. This decreases the load on database and network. Once a base QVD is generated, an incremental load script can ensure that only the new or updated records are saved to the QVD (and the deleted records are removed) when the script is made to run again. QVDs also save, reload time and space as they are both highly compressed and lightning fast when used as a load source in qlikview queries.
Why to use QVD?
There are many reasons for why one must use QVD, some of these are:
- Decoupling data extract from data presentation
- Ability to do parallel data extracts
- Easier unit testing of parts of the load process
- Incremental loads
- Sharing of extracted data between presentation apps
- Ability to scale the solution when data volumes grow
- Ability to delegate responsibility for different parts of the data load to different teams.
Related Page: OLE DB/ ODBC Connection To Data Sources In Qlikview
Advantages of QVDs
QVD files offer many advantages to your qlik view applications, including the following:
- Faster load times for large data sets: QVDs load many times faster than any other data source.
- Incremental loading of QVDs from an active and expanding database: Incremental loading is a sustainable way of loading data into your qlik view application, and it reduces the load time dramatically as compared to a full load.
- Less overhead on databases and networks: periodic loads from the data sources into the QVDs can be performed at off-peak times for shorter durations.
- Portability across qlik view applications: Each QVD can be used in many qlikview application binary loads (where entire data models and data sets from one QVW can be imported into another QVW) and can only be done once as per the qlik view application. An unlimited number of QVDs and other data sources can be used in any QVW
- Reduced file size of extracted data: Compared to an external file, a QVD file with millions of rows is fraction in size, due to the qlik view compression methodology.
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