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Referring back to On the previous page, here we shall now take a look at the log information, to help us understand how the VizQL engine is interpreting our calculations to SQL. From here, we can decide whether we can re-write the query to be more performant, or whether we can optimise the data source.I gave an overview of the logs; here I shall show you which logs to use, and what to look for to help you tune your workbooks.

I've previously explained that short of using the built-in functions such as Groups, Sets, Parameters, Aliases and Textboxes, all calculations are reinterpreted by the Tableau VizQL engine to the native language of your source; though despite the conversion, this isn't always the most optimum of queries. By using the information captured to the logs, we can see the query (eg SQL) that VizQL generated, copy the query to our data studio such as SQL Server Management Studio, and then re0execute the same code with Show Plan to see the effect of the query. 

Data source optimisation really is limited to a database such as Microsoft SQL Server, Teradata, Oracle etc; though there might still be opportunities to speed-up flat-file too.