https://pawelurbanek.com/explain-analyze-indexes

Interpreting the output of SQL EXPLAIN ANALYZE might seem like dark magic at first. I’m starting a series of blog posts where I’ll be well… explaining it based on common use cases from web applications. Read on if you’ve ever wanted to start using EXPLAIN ANALYZE for debugging slow PostgreSQL queries but did not manage to wrap your head around it. In the first part, we’ll discuss how the PostgreSQL query planner decides whether to use an index.

Examples in this tutorial are based on Ruby on Rails. However, SQL query analysis tips apply to all the web technologies like NodeJS, Python Django, or Elixir Phoenix.