# Database Indexing — System Design: Architecture, Scale and Trade-offs

Source: https://www.skillbyai.com/en/system-design/sd-indexing

> Speed up queries with B-tree indexes, understand the write cost, and order composite indexes for real query patterns.

## What an index is

An index is a sorted side-structure (usually a **B-tree**) pointing from a column's values to row locations, so the database can binary-search instead of scanning every row.

## The write cost

Every index must be updated on insert/update/delete, so more indexes mean slower writes and more storage. Index the columns you actually filter, join, or sort on — not everything.

## The book index

A database index is like the index at the back of a textbook: it costs pages (storage) and someone has to update it whenever content changes (writes), but it turns 'find every mention of X' from reading the whole book into one lookup.

## Composite index order matters

An index on `(country, city)` speeds up queries filtering by country alone or by country+city, but not by city alone — the leftmost column must be used. Order composite indexes by your actual query patterns.