QVD: Binary or not?

[ciaran_mcgowan]

Hi all,

I've always been lead to believe that QVD files were stored in binary format, making them more secure. I've recently been told that this in not actually the case but I've not seen any evidence to support this. Can anyone tell me definitively if this is true?

I'm aware that Qlikview stores field headers in XML format within the QVD but the data itself is still binary.

Any clarification would be greatly appreciated. Thank you.

[jonathandienst · Accepted solution]

There is no security or compression in a QVD file and anyone who suggests that does not know what they are talking about. Any user with QV or QS can open and view any QVD file. And you dont need a server or a license for that. You can even download QVD file viewers off the web.

Basically a QVD comprises:

• Metadata in XML, followed by
• The symbol table for each field (a list of the distinct values for each field), followed by
• The table body stored as bit stuffed pointers (using the bit count and offset stored in the metadata)

This is very efficient, but it is not compressed. The main advantage of QVD files is that they are an optimal storage form for QV/QS. The reload engine of these products can load the data from QVD files fatser than most other forms of storage, especially if the load is optimised.

For more, see 

https://community.qlik.com/blogs/qlikviewdesignblog/2012/11/20/symbol-tables-and-bit-stuffed-pointer...

Symbol Tables and Bit-Stuffed Pointers - a deeper look behind the scenes

[petter]

I mostly agree with you Jonathan. But in reality using a symbol table that is the term used by Qlik is the same as "Dictionary Compression" / "Library Compression" - so technically QVD files do use a very straight-forward and well-known compression technique.

Data Compression/Dictionary compression - Wikibooks, open books for an open world