One of the most common questions in software development interviews and database design meetings is: "What is the difference between a UUID and a GUID?"
The short answer? There isn't one. They are effectively the same thing. But the history, versions (v1, v4, v7), and performance implications for databases are worth understanding.
The Definition#
UUID stands for Universally Unique Identifier. It is an industry standard (RFC 4122).
GUID stands for Globally Unique Identifier. It is simply Microsoft's implementation term for the UUID standard.
If you are working in the Microsoft ecosystem (C#, .NET, SQL Server), you will see "GUID". In Java, Python, Rust, or PostgreSQL, you will see "UUID". They are binary-compatible, 128-bit integers displayed as 32 hexadecimal characters.
123e4567-e89b-12d3-a456-426614174000
xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
M = Variant (version)
N = Variant (layout)UUID Versions Explained#
While UUID and GUID are the same, the Version matters immensely.
- Version 1 (Time + MAC Address): Uses the current timestamp and your computer's MAC address. Guaranteed unique, but leaks privacy (your MAC address).
- Version 4 (Random): The most common standard. generated using cryptographically strong random numbers. Generate one here.
- Version 7 (Timestamp + Random): A newer standard (2024) designed for databases. heavily sortable by time, fixing fragmentation issues in SQL indexes.
Can UUIDs Collide?#
The fear of a "collision" (generating the same UUID twice) is mathematically irrational for v4 UUIDs.
There are 2122 possible UUIDs. That is 5.3 undecillions.
To have a 50% chance of a single collision, you would need to generate 1 billion UUIDs per second for 85 years. You represent a greater risk to your database reliability than a UUID collision does.
Performance: UUID as Primary Key#
Should you use UUIDs as database primary keys? It depends.
Pros:
- Obscures total record counts (unlike ID: 1, 2, 3)
- Allows offline generation (client creates ID before saving)
- Easy merging of databases/tables
Cons:
- Larger storage (16 bytes vs 4 bytes for INT)
- Slower indexing (fragmentation) compared to sequential INTs
- Harder to read/debug manually
Frequently Asked Questions
Are UUIDs case-sensitive?▼
Is a GUID larger than a UUID?▼
How do I generate a UUID in JavaScript?▼
Generate Secure UUIDs
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