Table Calculations are different from all other calculations in Tableau. Row-level, aggregate calculations, and LOD expressions, which we considered in the previous chapter, are performed as part of the query to the data source. If you were to examine the queries sent to the data source by Tableau, you'd find the code for your calculations translated into whatever flavor of SQL the data source used.

Table Calculations, on the other hand, are performed after the initial query. Here's an extended diagram that shows how aggregated results are stored in Tableau's cache:

Table Calculations are performed on the aggregate table of data in Tableau's cache right before the data visualization is rendered. As we'll see, this is important to understand for multiple reasons, including the following:

• Aggregation: Table Calculations operate on aggregate data. You cannot reference fields in a Table Calculation without referencing it as an aggregate.
• Filtering: Regular filters will be applied before Table Calculations. This means that Table Calculations will only be applied to data returned from the source to the cache. You'll need to avoid filtering any data necessary for Table Calculations to work as desired.
• Late Filtering: Table Calculations used as filters will be applied after the aggregate results are returned from the data source. The order becomes important: row-level and aggregate filters are applied first, the aggregate data is returned to the cache, then the Table Calculation is applied as a filter that effectively hides data from the view. This allows for some creative approaches to solving certain kinds of problems that we'll consider in some of the examples.
• Performance: If you are using a live connection to an enterprise database server, then row-level and aggregate-level calculations will be taking advantage of enterprise-level hardware. Table Calculations are performed in the cache, which means they will be performed on whatever machine is running Tableau. You will not likely need to be concerned if your Table Calculations are operating on a dozen or even hundreds of rows of aggregate data, or if you anticipate publishing to a powerful Tableau Server. However, if you are getting back hundreds of thousands of rows of aggregate data on your local machine, then you'll need to consider the performance of your Table Calculations. At the same time, there are cases where Table Calculations might be used to avoid an expensive filter or calculation at the source.