Hash Index
An index that uses a hash function to map keys directly to storage locations. O(1) lookups for exact matches but useless for range queries. Memcached and Redis use hash indexes.
What is Hash Index?
An index that uses a hash function to map keys directly to storage locations. O(1) lookups for exact matches but useless for range queries. Memcached and Redis use hash indexes.
Hash 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 "Hash Index" lesson linked below. It walks through the why, the mechanism, the trade-offs, and how the giants actually use it in production.
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See also
Related glossary terms you might want to look up next.
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.
B-Tree
A self-balancing tree data structure used by most relational databases for indexes. Keeps data sorted and allows searches, insertions, and deletions in O(log n).
Consistent Hashing
A hashing technique where adding or removing servers only moves a small fraction of keys. Used by Amazon DynamoDB and Cassandra for data distribution.