What internal statistics every self-respecting dbms should have!
MySQL has a lot internal statistical info — INFORMATION_SCHEMA, PERFORMANCE_SCHEMA, sys schema, Index Statistics and other. And though different dbms engines are designed in different way and so they have different statistics about internal processes and stuff, there's common pitfalls in metrics gathering. As they say "There are lies, damned lies, and statistics" I want to talk a little bit about these pitfalls and misleadings in performance stats in general.
I am a founder and CEO of SaaS monitoring solution Okmeter.io, where we try to implement monitoring with magic — just imagine a world where you actually have all the data about currently ongoing processes on your servers without need to configure and setup anything! That's what we're trying to achieve. Previously I worked as SRE team lead in largest russian internet job company - hh.ru. So I know a thing or two about monitoring different aspects of web-services, *nix systems, databases and stuff.