UUID vs ULID: Choosing the Right Unique ID Format
UUID v4 is the safe default — it is universally supported and has no predictable pattern. ULID is better when you need time-sortable IDs for database indexing, as it encodes a millisecond-precision timestamp in the first 10 characters, enabling natural chronological ordering.
UUID vs ULID — Feature Comparison
| Attribute | UUID | ULID |
|---|---|---|
| Format ULID is shorter and URL-safe by design. | 32 hex chars + 4 dashes (36 total) | 26 Crockford Base32 chars |
| Lexicographic sorting ULID sorts chronologically as a string, important for B-tree indexes. | Random — no natural order | Time-ordered (millisecond precision) |
| Database index performance | Poor — random UUIDs cause page splits | Good — sequential inserts are efficient |
| Collision probability | Extremely low (2^122 random bits) | Extremely low (80 random bits per ms) |
| Ecosystem support | Universal (SQL, NoSQL, all languages) | Growing (libraries for most languages) |
| Timestamp exposure ULID timestamp can be extracted — relevant for privacy-sensitive IDs. | No timestamp (v4) | Embeds creation timestamp |
| URL safety | Requires encoding if dashes stripped | URL-safe (no special chars) |
| RFC / standardisation | RFC 4122 + RFC 9562 | ULID spec (community standard) |
When to Use Each
Choose UUID when…
Use UUID v4 when you need universal compatibility and a well-established standard.
Choose ULID when…
Use ULID (or UUID v7) when database index performance and chronological ordering are important.
Frequently Asked Questions
Which is better for PostgreSQL primary keys: UUID or ULID?
ULID generally performs better for PostgreSQL primary keys because its time-ordered nature produces sequential inserts that avoid the B-tree page splits caused by random UUID v4. Alternatively, UUID v7 (introduced in RFC 9562) is a standardised time-ordered UUID that offers similar benefits.
Can I extract the timestamp from a ULID?
Yes. The first 10 characters of a ULID encode a Unix timestamp in milliseconds. This is useful for debugging and time-range queries, but means the creation time is embedded in the ID — consider this for privacy-sensitive applications.
Is ULID cryptographically secure?
ULID uses 80 bits of randomness (the last 16 characters), which is sufficient for uniqueness but less than UUID v4's 122 random bits. For cryptographic purposes or high-security tokens, prefer UUID v4.
What is UUID v7 and is it better than ULID?
UUID v7 (RFC 9562) is a standardised, time-ordered UUID format that encodes a Unix timestamp in the high bits. It offers similar database benefits to ULID while remaining fully UUID-compatible. UUID v7 is becoming the recommended choice for new systems where time-ordering matters.
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