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 at the data-source layer. 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, which 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. It's very important to understand this, for various reasons, including the following:
Aggregation: Table calculations operate on aggregate data. You cannot reference a field in a table calculation without referencing it as an aggregate.
Filtering: Regular filters will be applied...
