Why is my query slow? An indexing story

Indexes, the get out of jail card for slow queries am I right? I mean it worked, so should we care beyond that? Like in most things in software development, it depends. In this talk we will go on a journey and learn about: * What is an index and how does it work? * When should an index not be used? * What is the cost of indexing all the indexes? * Different types of indexes and when to use each? * How to debug why your index isn't being used?