How Do Database Indexes Speed Up Queries?

Database indexes are the difference between a query that completes in milliseconds and one that brings your application to its knees. After optimizing databases for over a decade—from small startups to systems handling billions of queries daily—I’ve learned that understanding indexes deeply is essential for building performant applications. A well-placed index can transform a 30-second query into one that completes in 10 milliseconds. This guide explains how indexes work internally and how to use them effectively in production.