OEG Best Practice: Dimension order
Dimension order affects calculation performance
Ensuring consistency in the order of dimensions will help improve the performance of your models. This consistency is relevant for modules and individual line items. Why does the order matter? Anaplan creates and uses indexes to perform calculations. Each cell in a module where dimensions intersect is given an index number.
Here are two simple modules dimensioned by Customer and Product. In the first module, Product comes first and Customer second, and in the second module, Customer is first and Product is second.
In this model, there is a third module that calculates revenue as Prices * Volumes.
Anaplan assigns indexes to the intersections in the module. Here are the index values for the two modules. Note that some of the intersections are indexed the same for both modules: Customer 1 and Product 1, Customer 2 and Product 2, and Customer 3 and Product 3, and that the remainder of the cells has a different index number. Customer 1 and Product 2 is indexed with the value of 4 in the top module and the value of 2 in the bottom module.
The calculation is Revenue = Price * Volume.
To run the calculation, Anaplan performs the following operations by matching the index values from the two modules.
Since the index values are not aligned, the processor scans the index values to find a match before performing the calculation.
When the dimensions in the module are reordered, these are the index values:
The index values for each of the modules are now aligned. As the line-items of the same dimensional structure have an identical layout, the data is laid out linearly in memory. The calculation process accesses memory in a completely linear and predictable way. Anaplan’s microprocessors and memory sub-systems are optimized to recognize this pattern of access and to pre-emptively fetch the required data.
How does the dimension order become different between modules? When you build a module, Anaplan uses the order that you drag the lists onto the Create Module dialog. The order is also dependent on where the lists are added. The lists that you add to the 'pages' area are first, then the lists that you add to the 'rows' area, and finally the lists added to the 'columns' area.
It is simple to re-order the lists and ensure consistency. Follow these steps:
- On the Modules pane, (Model Settings>Modules) look for lists that are out of order in the Applies To column. Click the Applies To row that you want to re-order, then click the ellipsis.
- In the Select Lists dialog, click OK.
- In the Confirm dialog, click OK.
- The lists will be in the order that they appear in General Lists.
- When you have completed checking the list order in the modules, click the Line Items tab and check the line items. Follow steps 1 through 3 to re-order the lists.
Subsets and line item subsets
One word of caution about Subsets and Line Item subsets. In the example below, we have added a subset and a Line Item Subset to the module:
The Applies To is as follows:
Clicking on the ellipsis, the dimensions are re-ordered to:
The general lists are listed in order first, followed by subsets and then line item subsets.
You still can reorder the dimensions by double-clicking in the Applies to column and manually copying or typing the dimensions in the correct order.
Largest vs. smallest?
This is the normal follow up question, and unfortunately, the answer is "it depends." Through research we have found that it all depends on the data within the module. Also, it can get very confusing if subsets are used; the Customer list might be bigger than the Products list, but if a subset of Customers is used that is smaller than Products, then what?
Also, we don't advocate ordering the lists in the General Lists setting in size order; the lists should be ordered in hierarchical order top to bottom, so, by definition, that will be smallest to largest. So our advice is be consistent. Think about how you describe the problem. Does the business talk about Customer by Product, or Products for Customers? Agree to a convention, and stick to it.
The calculation performance only relates to the common lists between the source(s) and the target. The order of separate lists in one or other doesn’t have any bearing on the calculation speed.
Author David Smith.