Difficulty: Advanced
Reading Time: 35 min read
Last Updated: September 01, 2025
Why Hash Indices?
In the last article, we explored Ordered Indexes and the B+-Tree — structures that shine when queries rely on order. They allow efficient range queries like:
- “Find all products priced between $100 and $500.”
- “List all usernames alphabetically between Alice and Bob.”
But sometimes… order doesn’t matter at all.
If your query is:
- “Find the record with ProductID = 30035”
- “Does username = 'Alice99' exist?”
…then walking down a multi-level tree is overkill. You don’t need sorted traversal — you just need the answer, instantly.
That’s where Hash Indices come in.
Hashing transforms a search key (e.g., ProductID) into a bucket address using a hash function. Instead of traversing nodes, the DB jumps straight to the location where the record should be.
👉 Ordered Indexes & B+-Trees → best for range queries.
👉 Hash Indices → best for point queries (exact match lookups).
Inside Hash Indexing
- Hash Functions → map search keys into buckets.
- Collisions → two different keys land in the same bucket. Handled with chaining or open addressing.
- Static Hashing → fixed buckets, fast but inflexible.
- Dynamic Hashing → buckets split/grow as data expands. (Extendible & Linear Hashing).
- Limitations → no ordering → range queries are useless.
Why It Matters
When the query is “find this exact record”, nothing beats hashing.
That’s why high-performance systems — from OLTP databases to key-value stores — rely heavily on hash indices for equality lookups.
Modern systems often combine hashing with Bloom Filters, which act as a quick pre-check for membership. While Bloom Filters may yield false positives, they guarantee no false negatives, making them perfect for filtering massive datasets.
Read the full article in my Notion blog here:
📌 Note:
The full article lives in my Notion blog here, which serves as the single hub for all my articles and ensures consistent formatting across platforms. You can read this article directly in the Notion link above. Feel free to share your thoughts or feedback in the site comments—or drop me a note on LinkedIn.
About the Author
Abdul-Hai Mohamed | Software Engineering Geek’s.
Writes in-depth articles about Software Engineering and architecture.
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