Why Isn't the Database Using My Index?

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?

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