Typical talks about database indexes work by showing simple queries that are slow and running EXPLAIN to add indexes. But nobody understands EXPLAIN output - its super complicated!

This talk will focus on stuff to understand why a (specific) index wasn't used from practical and theoretical knowledge with a ton of illustrations than obscure SQL commands.

Topics covered:

• An index can't be used: column transformations, multi-column index ordering, invisible indexes
• No index is best: small intro to how indexes are selected and why they are not used when reading many/few rows or why the database predicts wrong
• Another index is better: Your index wasn't used because the other one is predicted to be better. But why?