Chapter 19: Introduction to Index Design — How to Place Indexes That Speed Up Search, and the Pitfalls
This chapter covers, for junior engineers and focused narrowly on design-time decisions, how an index speeds up search (the intuition behind B-Trees), which columns to index (WHERE / JOIN / ORDER BY), composite-index column order, partial indexes, how to read EXPLAIN, the harm of over-indexing, and compatibility with UUID primary keys.