Index
A data structure that speeds up database lookups. Like the index at the back of a book that lets you jump to the right page instead of reading every page.
What is Index?
A data structure that speeds up database lookups. Like the index at the back of a book that lets you jump to the right page instead of reading every page.
Index is a foundational concept that sits in the Database Fundamentals area of system design. Engineers reach for it whenever they need to reason about real-world trade-offs in that space — not just for textbook correctness, but because real production systems at companies like Netflix, Amazon, and Google make these decisions every day.
If you want to go deeper than this definition — with diagrams, code, and a quiz to lock it in — work through the "Index" lesson linked below. It walks through the why, the mechanism, the trade-offs, and how the giants actually use it in production.
Learn Index in depth
Full interactive lesson with diagrams, code examples, real-world references, and a quiz.
Open the Index lessonRelated lessons
Lessons that touch on Index as part of a larger topic.
Database Indexing
B-trees, hash indexes, composite indexes, covering indexes, the single biggest performance lever in any database
foundation · database fundamentals
Index Rebuilding
When your database indexes rot and queries slow to a crawl, how to fix it without taking the system down
foundation · database fundamentals
Forward Index
The document-to-terms mapping that complements inverted indexes, how search engines store what each document contains
intermediate · database types storage
Inverted Index
The data structure powering every search engine, mapping words to the documents that contain them
intermediate · database types storage
Bitmap Index
Bit arrays that make low-cardinality queries blazing fast, the secret weapon of data warehouses
intermediate · database types storage
See also
Related glossary terms you might want to look up next.
SQL
Structured Query Language for managing relational databases. Tables, rows, columns, and powerful joins to query related data.
Database
An organized collection of data that can be easily accessed, managed, and updated. The backbone of almost every application.
NoSQL
Databases that don't use traditional table-based relational models. Includes document stores, key-value, graph, and column-family databases.