QlikView Reference Manual.pdf - QlikCommunity - QlikView

28 QVD Files
A QVD (QlikView Data) file is a file containing a table of data exported from QlikView. QVD is a native
QlikView format and can only be written to and read by QlikView. The file format is optimized for speed
when reading data from a QlikView script but it is still very compact. Reading data from a QVD file is typically
10-100 times faster than reading from other data sources.
QVD files can be read in two modes, standard (fast) and super-fast. The selected mode is determined automatically
by the QlikView script engine. Super-fast mode can be utilized only when all fields or a subset
thereof are read without any transformations (formulas acting upon the fields), though the renaming of fields
is allowed.
A QVD file holds exactly one data table and consists of three parts:
1. A well formed XML header (in UTF-8 char set) describing the fields in the table, the layout of
the subsequent information and some other meta-data.
2. Symbol tables in a byte stuffed format.
3. Actual table data in a bit-stuffed format.
28.1 Purpose of QVD Files
QVD files can be used for many purposes. At least four major uses can be easily identified. More than one
may apply in any given situation:
Increasing Load Speed
By buffering non-changing or slowly changing blocks of input data in QVD files, script execution becomes
considerably faster for large data sets.
Decreasing Load on Database Servers
The amount of data fetched from external data sources can also be greatly reduced. This reduces work load
on external databases and network traffic. Furthermore, when several QlikView scripts share the same data it
is only necessary to load it once from the source database. The other applications can make use of the same
data via a QVD file.
Consolidating Data from Multiple QlikView Applications
With the Binary (page 223) script statement it is limited to loading data from a single QlikView application
into another one, but with QVD files a QlikView script can combine data from any number of QlikView
applications. This opens up possibilities e.g. for applications consolidating similar data from different business
units etc.
Incremental Load
In many common cases the QVD functionality can be used for facilitating incremental load, i.e. exclusively
loading new records from a growing database.
To learn more about the use of QVD files and Incremental Load, follow the link below:
Using QVD Files for Incremental Load (page 394)
28.2 Create QVD Files
A QVD file can be created by one of three different methods:
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