---
title: "Ruby Hash Isn’t Always a Hash Table (And Why That Matters)"
url: https://daily.dev/posts/ruby-hash-isn-t-always-a-hash-table-and-why-that-matters--xz895trui
source_url: https://rubystacknews.com/2026/05/11/ruby-hash-isnt-always-a-hash-table-and-why-that-matters
type: article
source: "Ruby Flow"
published: 2026-05-12T09:07:54.787Z
updated: 2026-05-12T09:08:32.987Z
tags: ["performance", "ruby", "data-structures"]
reading_time: 4
upvotes: 1
comments: 0
language: en
---

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# Ruby Hash Isn’t Always a Hash Table (And Why That Matters)

**[Ruby Flow](https://daily.dev/sources/rubyflow)** · 4 min read · 1 upvotes · 0 comments

## Summary

Ruby's Hash is not a simple hash table — it uses a hybrid internal structure. For small hashes (≤8 entries), CRuby uses an AR (Array Representation) table stored inline with no heap allocation, using 1-byte hash hints for fast prefiltering before equality checks. Once a hash grows beyond the threshold, it automatically converts to a traditional ST table with open addressing and O(1) lookups. Key practical takeaways include: small hashes are extremely cheap, mutable keys cause unpredictable behavior, and custom key objects must implement both hash and eql? consistently. Ruby also guarantees insertion order since version 1.9.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://rubystacknews.com/2026/05/11/ruby-hash-isnt-always-a-hash-table-and-why-that-matters>

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---

Tags: [#performance](https://daily.dev/tags/performance), [#ruby](https://daily.dev/tags/ruby), [#data-structures](https://daily.dev/tags/data-structures)

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