Databases Reference
In-Depth Information
QVD and QVX files
We have now gone through the process of loading data from traditional databases
and simple table files. In this section, we will take a deeper look at the QVD and QVX
file types, which are used by QlikView to store and read data in an optimized format.
We will discuss a little more about both of these types, and the benefits and uses of
each of them.
QVD iles
QlikView Data (QVD)
files are used to extract and store data into and from
QlikView. This means that whichever table you read, from whichever database, you
can store it in the QVD format before or after any transformations you perform on
the table. The special characteristics of this file type are:
• It contains only one logical table.
• It uses a special algorithm to compress the data, achieving compression
rates of up to 90 percent, depending on the fields' cardinality of the
underlying data.
• When reading a QVD table file in QlikView, the loading speed is anywhere
from 10 to 100 times faster than when loading from a database. When the
table file is being read without applying any transformations, QlikView
performs an optimized load (super-fast mode).
One of the main advantages of using QVD files is that, once you have a QVD on your
disk, the table can be exploited by more than one QlikView application. This reduces
the load on the database server, and optimizes QlikView resources and development
time. This process is called QVD staging and is discussed further in
Chapter 12
,
Advanced Data Transformation
.
The QVD file will also be particularly useful when dealing with Incremental
Load scenarios which are discussed in
Chapter 12
,
Advanced Data Transformation
.
QVX files
QlikView data eXchange (QVX)
files are used for data input from external
systems into QlikView. The main difference with respect to the QVD file is that
QVX is a public format and can be created from external interfaces. It can be
considered as the format in which custom data sources (described earlier in the
chapter) send data to QlikView via the custom connector. Data retrieval becomes
optimized when complying with QVX specifications, although not as optimized
as QlikView's own QVD.
