If your app contains a large amount of data from database sources that are continuously updated, reloading the entire data set can be time consuming. In this case, you want to load new or changed records from the database. All other data should already be available in the app. Incremental load using QVD files, makes it possible to achieve this.
The basic process is described below:
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Load new or updated data from the database source table.
This is a slow process, but only a limited number of records are loaded.
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Load data that is already available in the app from the QVD file.
Many records are loaded, but this is a much faster process.
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Create a new QVD file.
This is the file you will use the next time you do an incremental load.
- Repeat the procedure for every table loaded.
The following examples show cases where incremental load is used. However, a more complex solution might be necessary, depending on the source database structure and mode of operation.
- Append only (typically used for log files)
- Insert only (no update or delete)
- Insert and update (no delete)
- Insert, update and delete
You can read QVD files in either optimized mode or standard mode. (The method employed is automatically selected by the Qlik Sense engine depending on the complexity of the operation.) Optimized mode is about 10 times faster than standard mode, or about 100 times faster than loading the database in the ordinary fashion.
For more information, see Working with QVD files.
Append only
The simplest case is the one of log files; files in which records are only appended and never deleted. The following conditions apply:
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The database must be a log file (or some other file in which records are appended and not inserted or deleted) which is contained in a text file (ODBC, OLE DB or other databases are not supported).
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Qlik Sense keeps track of the number of records that have been previously read and loads only records added at the end of the file.
Example:
Buffer (Incremental) Load * From LogFile.txt (ansi, txt, delimiter is '\t', embedded labels);
Insert only (no update or delete)
If the data resides in a database other than a simple log file, the append approach will not work. However, the problem can still be solved with a minimum amount of extra work. The following conditions apply:
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The data source can be any database.
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Qlik Sense loads records inserted in the database after the last script execution.
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A ModificationTime field (or similar) is required for Qlik Sense to recognize which records are new.
Example:
QV_Table:
SQL SELECT PrimaryKey, X, Y FROM DB_TABLE
WHERE ModificationTime >= #$(LastExecTime)#
AND ModificationTime < #$(BeginningThisExecTime)#;
Concatenate LOAD PrimaryKey, X, Y FROM File.QVD;
STORE QV_Table INTO File.QVD;
The hash signs in the SQL WHERE clause define the beginning and end of a date. Check your database manual for the correct date syntax for your database.
Insert and update (no delete)
The next case is applicable when data in previously loaded records may have changed between script executions. The following conditions apply:
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The data source can be any database.
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Qlik Sense loads records inserted into the database or updated in the database after the last script execution.
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A ModificationTime field (or similar) is required for Qlik Sense to recognize which records are new.
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A primary key field is required for Qlik Sense to sort out updated records from the QVD file.
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This solution will force the reading of the QVD file to standard mode (rather than optimized), which is still considerably faster than loading the entire database.
Example:
QV_Table:
SQL SELECT PrimaryKey, X, Y FROM DB_TABLE
WHERE ModificationTime >= #$(LastExecTime)#;
Concatenate LOAD PrimaryKey, X, Y FROM File.QVD
WHERE NOT Exists(PrimaryKey);
STORE QV_Table INTO File.QVD;
Insert, update and delete
The most difficult case to handle is when records are actually deleted from the source database between script executions. The following conditions apply:
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The data source can be any database.
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Qlik Sense loads records inserted into the database or updated in the database after the last script execution.
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Qlik Sense removes records deleted from the database after the last script execution.
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A field ModificationTime (or similar) is required for Qlik Sense to recognize which records are new.
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A primary key field is required for Qlik Sense to sort out updated records from the QVD file.
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This solution will force the reading of the QVD file to standard mode (rather than optimized), which is still considerably faster than loading the entire database.
Example:
Let ThisExecTime = Now( );
QV_Table:
SQL SELECT PrimaryKey, X, Y FROM DB_TABLE
WHERE ModificationTime >= #$(LastExecTime)#
AND ModificationTime < #$(ThisExecTime)#;
Concatenate LOAD PrimaryKey, X, Y FROM File.QVD
WHERE NOT EXISTS(PrimaryKey);
Inner Join SQL SELECT PrimaryKey FROM DB_TABLE;
If ScriptErrorCount = 0 then
STORE QV_Table INTO File.QVD;
Let LastExecTime = ThisExecTime;
End If