Why UUIDs Might Not Be the Best Choice for Database Keys (And What You Can Use Instead)

When designing databases, choosing the right primary key is crucial for performance and scalability. One popular option for primary keys is the UUID (Universally Unique Identifier). While UUIDs are great for ensuring uniqueness across distributed systems, they might not always be the best choice when indexing databases.

In this article, we'll explore why UUIDs can be a bad choice for primary keys, especially when indexing, and suggest alternatives like ULIDs and traditional auto-incrementing keys.

What is a UUID?

A UUID is a 128-bit identifier designed to be unique across space and time. It’s commonly used in distributed systems where generating unique keys without centralized control is important.

Here’s an example of a UUID:

This format ensures that even if two systems generate UUIDs simultaneously, the chances of collision are almost zero.

Why UUIDs Might Not Be the Best Choice for Primary Keys

Despite their uniqueness, UUIDs can cause several issues when used as primary keys in databases, especially when indexing:

1. Performance Issues with Indexing

UUIDs are randomly generated, meaning they don’t follow any particular order. In a typical B-tree or other index structures, inserting new values is optimized for sequential or ordered data. UUIDs break this pattern by causing inserts to happen at random locations in the index, leading to:

• Fragmentation: Indexes become fragmented because new entries are inserted in non-sequential order.
• Cache Misses: Non-sequential inserts mean the data is scattered, leading to more cache misses and slower access times.
• Slower Writes: The more fragmented the index, the slower the write performance, especially with large data sets.

2. Increased Storage Overhead

A UUID is 128 bits long (16 bytes), whereas traditional auto-incrementing keys are typically 4 or 8 bytes. This means that using UUIDs will require more storage space, not just for the key itself but also for every index that includes the key.

3. Harder to Debug

UUIDs are long and not human-readable, making it harder to debug or reference specific records during development or troubleshooting.

Alternatives to UUIDs for Primary Keys

1. ULIDs (Universally Unique Lexicographically Sortable Identifiers)

ULID is a more recent alternative that aims to address some of the downsides of UUIDs. ULIDs are also 128-bit but come with a few key advantages:

• Lexicographically Sortable: ULIDs are designed to be sortable based on creation time. This means they work better with index structures since newer values will be inserted in sequence.
• Human-Friendly: ULIDs are more readable and often shorter than UUIDs, making them easier to work with during debugging.

Here’s an example of a ULID:

Since they are lexicographically ordered, they avoid the random insertion problem associated with UUIDs and provide better write performance in databases.

2. Traditional Auto-Incrementing Keys

For many applications, the simplest solution is often the best: auto-incrementing keys.

With auto-incrementing primary keys, each new record gets a sequential integer value as its primary key, like:

Advantages of auto-incrementing keys:

• Simple and Efficient: They are small (usually 4 or 8 bytes) and fit neatly into database indexes, leading to optimal read and write performance.
• Sequential Inserts: New records are always appended at the end of the index, minimizing fragmentation and making indexing highly efficient.
• Human-Readable: Debugging and referencing records is much easier with small, sequential integers.

However, the main drawback is that they are not globally unique across distributed systems. If you’re working in a single-database environment, this might not be an issue, but in distributed systems, you may need to handle potential key collisions carefully.

Which One Should You Use?

• For Distributed Systems: If you need globally unique identifiers and are working in a distributed system, consider ULIDs over UUIDs due to their sortability and better indexing behavior.
• For Simple Applications: If you’re working with a single database or don’t require global uniqueness, a traditional auto-incrementing key will likely be the most efficient option in terms of performance and storage.

Conclusion

While UUIDs are a popular choice for ensuring uniqueness, they come with performance trade-offs when used as primary keys, particularly in terms of indexing and storage efficiency. Alternatives like ULIDs provide a more index-friendly solution, while auto-incrementing keys are a solid choice for simpler applications.

Choosing the right key depends on your use case, but understanding the trade-offs between these options will help you make a more informed decision.