When you start tuning a Tableau dashboard or SQL query for performance, one thing jumps out quickly: COUNTD can slow things down.
The count-distinct is an extremely useful function which looks simple on the surface, but under the hood it forces the engine to do extra work that adds up fast. Understanding why this happens – and what alternatives you can use – makes a big difference in keeping your dashboards responsive.
[Read more…] about Why is COUNTD so slow – and how can I fix this?