Delta Load, sometimes called Incremental Load, is the technique of pulling only changed or new rows from a database and then merging those rows with a master QVD. The Delta Load pattern follows these general steps:
1. Determine high water mark (“last reload”)
2. Build WHERE predicate in syntax of target DB.
3. SQL SELECT delta rows.
4. Merge delta rows with QVD.
5. If Deletes, INNER JOIN entire set of Keys from DB with QVD
The first step is to determine what cutoff value — delta point — do we want to pass in the SQL SELECT WHERE clause to identify new rows. This value is usually the highest value in the existing QVD.
The most robust and reliable method for step one is loading with max() from the existing QVD. For example:
LOADmax(LastModified) asMaxModifiedFROMmyqvd.qvd (qvd);
This works well, but reading the entire QVD can take a very looooong time for a large QVD.
A much faster method is to aggregate the max value for the Delta rows only and then save that value on the first row of the QVD. In subsequent delta loads, only the first row of the QVD is read to get the value. This is extremely fast and is not effected by QVD size. For example:
// Do QVD Merge…and STORE Facts INTO Facts.qvd;
The “HighWaterValue” field will contain a relatively small number of values and will therefore have a negligible impact on the size of the QVD. You will of course have to create the field the first time before trying to load it with the “FIRST 1…”.
If you are using Qlikview Components (QVC) V8+ for delta loading, you can enable this high performance strategy by setting the variable:
QVC will automatically create a high water field named Qvc.MaxModFieldValue
and detect the availability of the field on subsequent loads. There is no need to pre-create the field before turning on this feature.
The technique is part of the Advanced Scripting material I’ll be presenting at the upcoming Masters Summit for Qlikview in Europe this October. I’ll be discussing more about Delta Loads, including strategies for merging large QVDs and tables with non-unique keys. Review the full agenda here and decide if the Masters Summit might help take your Qlikview skills to the next level.