UUID v4 vs v6: What's the Difference?

Introduction

UUIDs (Universally Unique Identifiers) are 128-bit numbers used to uniquely identify information in distributed systems. UUID v4 and v6 are two popular versions with different characteristics and use cases.

This guide explains the key differences to help you choose the right UUID version.

UUID v4 (Random)

UUID v4 generates completely random identifiers:

550e8400-e29b-41d4-a716-446655440000
  f47ac10b-58cc-4372-a567-0e02b2c3d479
  3d6f4e2a-9b7c-4a1f-8e5d-2c3b4a5f6e7d

Generation: Uses cryptographically secure random numbers
Pros: Simple, no coordination needed, highly secure
Cons: Poor database performance, not sortable
Uniqueness: Practically guaranteed (collision chance: 1 in 2^122)

UUID v6 (Time-Ordered)

UUID v6 includes timestamp for better sorting and database performance:

1ef3c8f0-8b2a-6000-8000-00005ab8e4c0
  1ef3c8f1-2d4e-6000-8000-00005ab8e4c1
  1ef3c8f2-7f9a-6000-8000-00005ab8e4c2

Generation: Timestamp + random bits
Pros: Sortable, better DB performance, maintains chronological order
Cons: Slightly more complex generation
Structure: Time-high + time-mid + time-low + clock-seq + node

Performance Comparison

Metric	UUID v4	UUID v6
Database Index Performance	Poor (random insertions)	Excellent (sequential)
Sortability	Not sortable	Chronologically sortable
Generation Speed	Very fast	Fast
Collision Risk	Negligible	Negligible
Storage Size	128 bits	128 bits

When to Use UUID v4

Maximum privacy: No time information revealed
Simple requirements: Don't need sorting
Distributed systems: Multiple servers generating IDs
Security tokens: Session IDs, API keys
Small scale: Database performance not critical

When to Use UUID v6

Database primary keys: Better index performance
High-volume inserts: Millions of records
Time-series data: Events, logs, transactions
Need sorting: Display by creation time
Distributed databases: CockroachDB, YugabyteDB

Database Impact

UUID v4 causes random B-tree insertions, leading to:

Frequent page splits
Poor cache locality
Slower writes (2-3x slower than v6)
Index fragmentation

UUID v6 provides sequential insertions:

Minimal page splits
Better cache usage
Faster writes
Compact indexes

Code Examples

// UUID v4 (JavaScript)
  const uuid = crypto.randomUUID();
  // Output: '9b1deb4d-3b7d-4bad-9bdd-2b0d7b3dcb6d'

  // UUID v6 (using library)
  import { v6 as uuidv6 } from 'uuid';
  const uuid = uuidv6();
  // Output: '1ef3c8f0-8b2a-6000-8000-00005ab8e4c0'

  // PostgreSQL
  CREATE TABLE users (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    created_at TIMESTAMP DEFAULT NOW()
  );

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