What is the leftmost prefix rule in composite indexes?

In a composite index on (A, B, C), MySQL can use the index for queries filtering on (A), (A, B), or (A, B, C), but cannot use it if filtering only on (B) or (C).

MySQL & Databases

Database Indexing Strategies: B-Trees, Composite Indexes & Query Plans

Understand how B-Tree indexes work under the hood, how the leftmost prefix rule governs composite indexes, and how EXPLAIN reveals bottlenecks.

Written by Marcus Thorne

Published: 10/06/2026 8 min read

Unlocking Fast Relational Queries

Indexes are lookup tables that the database search engine can consult to locate records without scanning every row in a table. Use EXPLAIN before queries to verify that MySQL uses targeted index lookups rather than full table scans (type: ALL).

Marcus Thorne

Head of Technology & Web Engineering

Marcus is a full-stack engineer and educator who has architected fintech payment gateways, secure authentication engines, and performant data pipelines using modern PHP and MySQL.

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