Approximate functions: How do they work?

This page contains the description for my conference session “Approximate functions: How do they work?”, and links to the slide deck and demo code used in this presentation.


Target audience Experienced database developers and DBAs, plus all curious geeks.
Short description When a close approximation is good enough, “Approximate Query Processing” can save you a lot of performance. But how? Find out by attending this internals deep dive session!
Duration The ideal length for this session is 60 – 75 minutes.
Full abstract Sometimes, a close approximation is good enough. And sometimes, a close approximation is a lot faster. Microsoft has introduced “Approximate Query Processing” (the APPROX_COUNT_DISTINCT and APPROX_PERCENTILE functions) to give you exactly that benefit when you don’t need exact answers.

But do you have a good response when you propose to use this function and your manager asks you to explain how they work first? Or is your only option to claim “black magic by smart Microsoft engineers”?

The algorithms used are not a secret. HyperLogLog and KLL Sketch. And now you most likely know exactly as much as you already knew before. And when you google for those terms … you end up with a headache.

Time to join me for a session where I explain the black magic in the simplest possible terms, so that you can then explain it to your manager!


This session has not yet been presented. I am still working on the slide deck and the demos, and will probably continue to do so until shortly before it is first presented. The “final” version of the slide deck and demo code will be uploaded here around the time I first deliver this session.

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