Why it matters

Hash indexes are underused. For high-QPS equality lookups (session lookup, user lookup by ID), they can outperform B-trees. Understanding when to use them is real optimization skill.

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The architecture

Structure: hash function maps key to bucket. Bucket holds pointer to row.

Lookup: hash key, follow pointer. Constant time.

Hash index propertiesHash key→ bucketBucket to rowpointerNo orderingno rangesEquality-only; use B-tree for anything needing ordering or range
Hash index trade-offs.
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How it works end to end

Collision handling: chaining or open addressing. Same as any hash table.

Resize: when hash table grows, may need rebuild.

Limitations: no range queries, no sorted iteration, no partial-key queries.