Understanding data modeling within QlikView is crucial for efficient application development and performance. Two key concepts that often arise are synthetic keys and link tables. While both relate to how QlikView handles data associations, they serve different purposes and have distinct implications. This article will delve into a detailed comparison of synthetic keys and link tables, clarifying their differences and guiding you on best practices for managing them in your QlikView applications.
Understanding Synthetic Keys in QlikView
A synthetic key emerges in QlikView when two or more tables have multiple fields with the same names. QlikView’s associative engine automatically attempts to link tables based on common field names. However, when multiple common fields exist across several tables, QlikView may create a synthetic key to manage these complex associations.
Synthetic keys are essentially composite keys generated by QlikView behind the scenes. While they enable data association, they are generally considered undesirable in most scenarios due to several drawbacks:
- Performance Overhead: Synthetic keys can be resource-intensive, potentially slowing down calculations and impacting application performance, especially with large datasets.
- Complexity and Maintainability: They make the data model more complex and harder to understand and maintain. Debugging and troubleshooting can become challenging with numerous synthetic keys.
- Data Interpretation Issues: In certain situations, synthetic keys might lead to unexpected data interpretations if not handled carefully.
It’s important to identify and, wherever possible, eliminate synthetic keys to ensure optimal QlikView application performance and maintainability. QlikView Desktop usually flags synthetic keys, making them visible in the data model viewer.
Exploring Link Tables (Central Link Tables) in QlikView
In contrast to synthetic keys, link tables, also known as central link tables, are a deliberate design pattern used in QlikView to manage relationships between multiple fact tables that share common dimensions. They are particularly useful in star schema data models where you have multiple fact tables connected to shared dimension tables.
Instead of QlikView automatically creating synthetic keys when fact tables share dimensions, you can proactively create a central link table. This involves concatenating the dimension fields from the related tables into a single link table. This central link table then acts as an intermediary, linking back to the fact tables on one side and dimension tables on the other.
The benefits of using link tables are significant:
- Simplified Data Model: Link tables simplify the data model by centralizing dimension relationships, making it easier to understand and manage.
- Improved Performance: By explicitly creating link tables, you often avoid complex synthetic keys, leading to improved query performance and faster calculations.
- Enhanced Flexibility: Link tables provide greater flexibility in data modeling and can facilitate more complex data analysis scenarios.
Synthetic Keys vs. Link Tables: Key Differences Summarized
| Feature | Synthetic Key | Link Table (Central Link Table) |
|---|---|---|
| Creation | Automatically generated by QlikView | Manually designed and created |
| Purpose | Implicitly manages complex associations | Explicitly manages relationships between fact tables and dimensions |
| Desirability | Generally undesirable due to performance and complexity | Desirable and a best practice for certain data models |
| Impact | Potential performance issues, complex model | Improved performance, simplified model, enhanced flexibility |
| Control | Limited direct control | Full control over design and implementation |
Avoiding and Resolving Synthetic Keys
While link tables are a design choice, avoiding synthetic keys is generally a best practice. Here are common methods to prevent and resolve synthetic keys in QlikView:
- Field Renaming: The simplest approach is to rename fields in your load script so that common fields across tables have distinct names. This breaks the condition for synthetic key creation.
- Unqualify Operator: The
UNQUALIFYoperator can be used to remove the table name qualification from specific fields, preventing QlikView from recognizing them as common fields across tables when they are not intended to be linked directly. - Qualify Operator (Used Strategically): Conversely, the
QUALIFYoperator can be used to explicitly qualify field names, ensuring that only intended common fields are considered for association.
In some specific cases, such as when using IntervalMatch, synthetic keys might be unavoidable and even expected. However, for most standard data modeling scenarios, striving to eliminate synthetic keys through the methods mentioned above is recommended.
Conclusion
Synthetic keys and link tables represent contrasting approaches to data association in QlikView. Synthetic keys, often automatically generated and generally undesirable, highlight potential issues in data modeling. Link tables, on the other hand, are a deliberate and beneficial design pattern for managing complex relationships, particularly in scenarios with multiple fact tables. By understanding the nuances of both, and prioritizing the avoidance of synthetic keys while strategically employing link tables, you can build robust, performant, and maintainable QlikView applications. Choosing the right approach significantly impacts the efficiency and clarity of your QlikView data model.
