UUID vs ULID — Which to Use?
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Open UUID Generator →What is a UUID?
A UUID (Universally Unique Identifier) is a 128-bit identifier standardised in RFC 4122. The most common version, UUID v4, is randomly generated — making collisions astronomically unlikely.
550e8400-e29b-41d4-a716-446655440000
A UUID has 32 hex characters split into 5 groups by hyphens. The format is: 8-4-4-4-12.
What is a ULID?
A ULID (Universally Unique Lexicographically Sortable Identifier) is also 128 bits, but combines a 48-bit millisecond timestamp with 80 bits of randomness.
01ARZ3NDEKTSV4RRFFQ69G5FAV
ULIDs use Crockford's Base32 encoding — 26 characters, uppercase letters and digits, no hyphens. The first 10 characters encode the timestamp, making ULIDs sortable by creation time.
UUID vs ULID — key differences
| Feature | UUID v4 | ULID |
|---|---|---|
| Length | 36 chars (with hyphens) | 26 chars |
| Sortable by time? | No | Yes |
| Contains timestamp? | No | Yes (ms precision) |
| URL safe? | Yes | Yes |
| Case sensitive? | Lowercase convention | Case insensitive |
| Standardised? | RFC 4122 | Community spec |
| DB index performance | Random — poor for sequential inserts | Sorted — better for indexes |
| Language support | Universally supported | Library required |
Database performance — the real difference
This is where ULIDs have a significant advantage. Database indexes (especially B-tree indexes like PostgreSQL's default) work best with sequential values. Random UUIDs cause index fragmentation — each new insert lands in a random position in the index, causing frequent page splits and increased I/O.
ULIDs, being time-sortable, are mostly inserted at the end of the index (since new records have larger timestamps). This leads to better write performance at scale — similar to auto-increment integers but without requiring a centralised counter.
When to use UUID
- When you need maximum compatibility — every language and database supports UUID natively
- When you don't want creation time to be inferable from the ID
- When working with systems that already use UUIDs
- For low-to-medium volume applications where index performance isn't a concern
When to use ULID
- High-volume write workloads where index performance matters
- When you want IDs that are naturally sortable by creation time
- When you want shorter, more readable IDs
- Event sourcing, logging systems, and audit trails
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