Introduction
QVD is a file with the extension (.qvd). It stores the data of a single table; that means that each row shares the same column name. It is typically created in a load script of a QVD file. The structure of a QVD file is essentially XML format. QVD files of QlikView store data. QVD files are used to provide a logical data solution in a QlikView solution. In some cases, QVD files avoid the use of a data warehouse.
QVD files hold only one single table data and also consists of the following three parts:
- The QVD files consist of well a formed XML header. The XML header describes the available fields in the table and also contains the layout of the information.
- Symbol tables in byte stuffed format.
- Actual table in bit stuffed format.
The QVD files can be read in two modes.
Standard mode
The standard mode is determined automatically by the QlikView script engine.
Optimized mode
The optimized mode is determined only when all loaded files are read without any transformation.
Purpose of QVD files
There are the following basic purposes of QVD files.
- The main purpose of a QVD file is to increase load speed. By unchanging and slowly changing blocks of input data in QVD files, the script execution becomes much faster for large data sets.
- The second purpose of QVD files is incremental load. In this QVD files loads new data from a growing database.
- The third and main purpose of the QVD file is decreasing the load on the database server, which means that when a huge amount of data is imported from an external data source then there is a heavy load generated on the database server. So by using QVD files we decrease the load on the database server.
- By using QVD files the QlikView script can combine data from any number of QlikView applications. QVD files consolidate data from different and multiple QlikView applications.
Creation of QVD
QVD files are created during the execution of a QlikView load script. QVD file syntax uses a STORE statement because store statements write current contents of a single table in the data model to a file. It is stored on a disk.
Syntax of QVD file
The following is the syntax to create a QVD file.
STORE TableName INTO ..\Data\FileName.qvd (QVD);
Use of QVD files
The following are the multiple uses of QVD files.
- With the use of QVD files, the incremental load can be adopted.
- From data presentation, complex data can be extracted.
- With the use of QVD files, parallel data can be extracted.
- Unit testing will be easy in the heavy load process.
- Data sharing also happens in data presentation.
How to use QVD files in the QlikView application
TableName:
LOAD
FieldList
FROM ..\Data\FileName.qvd (qvd);
Summary
This article introduced QlikView files and also described the syntax and use of QVD files in a QlikView application.