Ask the BW Expert: Restrictions versus Filters - What's the Difference?

  • by Ned Falk, Senior Education Consultant, SAP
  • July 1, 2004
Confusion continues to exist about restricting data when defining variables. The author clarifies this topic with definitions and examples. He also provides a test that you can try on your own system to confirm his explanations.

Dear BW Expert:

We are on BW 3.1 and our queries are defined with the BEx Query Designer. We define our variables in the Columns section of the BEx Query Designer rather than under Filters or Free Characteristics.

I always thought that when users specify a variable defined under the BEx Query Designer Filters, it was executed as a “real” filter, meaning that the OLAP processor extracted the data using that variable and any other filters. The front-end tool, then, applied the other variables and displayed the results.

A colleague recently told me that this is not what is happening. According to my colleague, the OLAP processor extracts all the data using the fixed filters and it is the front-end tool that applies the variables. Let me tell you, I have a hard time believing that’s true!

I was also under the impression that no variables are applied if they are added to the Free Characteristics section of the BEx Query Designer. Is that always the case? Even if you use Type H readings for queries in the Read only during navigation / hierarchy expand mode? Now, I hear that for variables, BW retrieves all of the data and then asks the users which subset they want to work with. Is that true!?

While we are on the subject of filtering, I’d like to ask you about navigational attributes. What happens if a query contains a navigational attribute in its free characteristics that is used for filtering only? Does the report display the pure attributes?

Guylaine Bélisle, Hydro Québec

 

Restricting data using variables is a core concept that I teach in all my front-end classes. It is a great way to enhance overall system performance. Although many query designers practice this basic design technique, most are not aware of why they do it and how it works.

Allow me to give you a quick answer to your questions and then I’ll elaborate further and provide you with some concrete proof. Fixed filters and variables are applied to the selection from the database server. Both take nearly an equal amount of time to process with the exception of variables for which the system must first derive a value if it is not supplied by the end user. These may take a fraction of a second longer. Let me quickly add that if custom ABAP is used to derive the variable, all bets are off!

I’ll show you the benefits variable-based restrictions offer to users of well-designed queries and share some convincing evidence for my answers, including those regarding navigational attributes. Most of my advice is general enough to pertain to any implementation of the BW.

Ned Falk

Ned Falk is a senior education consultant at SAP. In prior positions, he implemented many ERP solutions, including SAP R/3. While at SAP, he initially focused on logistics. Now he focuses on SAP HANA, SAP BW (formerly SAP NetWeaver BW), SAP CRM, and the integration of SAP BW and SAP BusinessObjects tools. You can meet him in person when he teaches SAP HANA, SAP BW, or SAP CRM classes from the Atlanta SAP office, or in a virtual training class over the web. If you need an SAP education plan for SAP HANA, SAP BW, BusinessObjects, or SAP CRM, you may contact Ned via email.

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