Glossary

Index (MySQL)

An index in MySQL is a separate data structure (usually a B-Tree) that lets the database find rows without scanning the entire table. Proper indexes are the fastest way to speed up slow queries — sometimes by orders of magnitude.

When an index helps

Columns in WHERE, JOIN ON, ORDER BY, GROUP BY
High-cardinality columns (many unique values) — ideal for email, UUID

When an index hurts

Tables with frequent INSERT/UPDATE/DELETE — every change updates the index
Low-cardinality columns (boolean, a status with 3 values) — MySQL may choose a full scan over the index

Composite indexes

An index on (a, b, c) is used left to right: a query with WHERE a = 1 AND b = 2 uses the index, but WHERE b = 2 alone does not. This is the leftmost prefix rule.