UUIDs are practically collision-resistant, not mathematically guaranteed to be unique. For a new distributed system that benefits from sortable database keys, UUIDv7 is a strong candidate; UUIDv4 remains appropriate when random ordering is preferred and its index-locality trade-off is acceptable. The right choice depends on how your database orders UUIDs and how your workload writes them.
What is a UUID, and are UUIDs truly unique?
A UUID is a 128-bit identifier. The name “universally unique” describes its practical purpose, not a mathematical guarantee that collisions are impossible. The IETF’s RFC 9562 explains that true global uniqueness cannot be guaranteed without shared knowledge, but that UUIDs can provide uniqueness for practical implementations without such a scheme.
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Different UUID versions reserve bits for version and variant information, leaving fewer than 128 bits for other data. UUIDv4 has 122 bits available for random data. That makes accidental collisions extraordinarily unlikely at ordinary scales, but not impossible. Whether a collision is acceptable depends on its consequences in your system: a database uniqueness constraint can detect one, but your application still needs a way to handle the conflict.
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Which UUID version should I use?
For a new distributed application whose database benefits from time-sortable keys, evaluate UUIDv7 first. It supports decentralized generation and places a Unix timestamp in the leading portion of the identifier, so UUIDs can sort by time when compared as raw bytes. UUIDv4 is a reasonable alternative when random ordering is desirable, when timestamp-related ordering leakage is a concern, or when your database workload makes index locality immaterial.
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| Version | How it is used | When to consider it |
|---|---|---|
| UUIDv4 | Random or pseudorandom identifier; 122 random bits per RFC 9562. | When you want random ordering, or random index inserts are acceptable for your engine and workload. |
| UUIDv7 | Unix timestamp plus additional random and/or counter fields; designed for time ordering. | A candidate for new distributed systems that benefit from sortable identifiers and database index locality. |
| UUIDv6 | Time-ordered format designed to make v1-style timestamps sort as bytes. | When a time-ordered UUID is wanted with v1-style timestamp behavior. |
| UUIDv1 | Timestamp and node-oriented identifier. | Only when its timestamp/node behavior is deliberately needed; RFC 9562 notes MAC-address privacy concerns. |
| UUIDv3 and UUIDv5 | Deterministic, name-based identifiers. | When the same namespace and name should produce a repeatable identifier, not as a substitute for an immutable surrogate key when the name is a changeable business value. |
Make the decision against the needs of your system, not the version number alone. Check whether identifiers must be generated independently across machines, how your database orders them, whether timestamp information is sensitive, the quality of your generator, the cost of a collision, and compatibility with existing APIs and schemas.
Is UUIDv7 better than UUIDv4 for database indexes?
It can improve locality in databases that sort UUIDs by their bytes, but it is not a universal performance fix. A B-tree index organizes keys in order. UUIDv4 values are random, so successive inserts can land at many different positions in the tree rather than clustering near the right edge. That can reduce locality and add work to common index structures.
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UUIDv7 puts time in the leading fields, making values generated around the same time sort near one another under byte-wise comparison. RFC 9562 says time-ordered monotonic UUIDs benefit from greater database-index locality because new values are near one another in the index. This is a design property, not a promise of a specific speedup: the result depends on the database engine, its UUID ordering, write pattern, indexes, and workload.
Do not assume every database compares UUIDs the same way. For example, Microsoft documents that SQL Server’s uniqueidentifier ordering is not implemented by comparing bit patterns (Microsoft Learn: uniqueidentifier). Therefore, the RFC’s raw-byte sorting behavior alone does not establish how UUIDv7 will behave in a SQL Server index. Confirm ordering semantics for your engine and release, then measure with your own write workload before migrating on the assumption that UUIDv7 will be faster.
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Does PostgreSQL support UUIDv7?
PostgreSQL 18 documents a native uuid type and native uuidv4() and uuidv7() generators. Its UUID type stores the value in native 128-bit form and can store any UUID version. PostgreSQL 18’s UUIDv7 generator uses a Unix timestamp with millisecond precision, along with sub-millisecond timestamp and random data. See the PostgreSQL 18 UUID type documentation and UUID functions documentation.
This is a version-specific statement, not a claim that every PostgreSQL release has a built-in UUIDv7 generator. Check the documentation for the release you actually run. A database may still store UUIDv7 values supplied by an application or extension even if its own release does not provide a native generator.
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Why use a native UUID column instead of text?
When the database offers a native UUID type, use it rather than storing UUIDs as text unless you have a concrete reason to keep a textual representation. The UUID itself is 128 bits; RFC 9562 notes that its text representation uses 288 bits. Native storage avoids carrying that extra textual representation in the database and lets the engine apply UUID-aware type and comparison behavior.
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What privacy and security trade-offs should you consider?
UUIDv7 includes a time component, which may reveal an identifier’s approximate creation time. That may be harmless for an internal key but undesirable when identifiers are exposed publicly and creation timing is sensitive. UUIDv4 does not encode a timestamp in the same way, although its use does not by itself make an application secure.
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Do not use a UUID as an authorization secret or assume that a UUID version guarantees unpredictability. For random values that need to resist prediction, RFC 9562 recommends a cryptographically secure pseudorandom number generator. Authorization should rely on appropriate access controls and purpose-built secrets, not merely on an identifier that is hard to guess.
Quick Recap
How to choose and validate a UUID strategy
- Define the identifier’s job. Decide whether it is a database key, a public reference, a repeatable name-based value, or a security-sensitive secret. One identifier should not be assumed to serve all of these roles.
- Check the database’s ordering rules. Confirm how the exact database engine and version compares and indexes UUIDs. Do not infer engine behavior from the UUID standard alone.
- Match the version to the trade-offs. Consider UUIDv7 for time sorting and potential locality in byte-sorting databases; consider UUIDv4 when random ordering is preferred and its write pattern is acceptable. Use other versions only where their specific timestamp or deterministic behavior is needed.
- Check privacy, generator quality, and compatibility. Account for timestamp exposure, use a cryptographically secure generator where unpredictability matters, and confirm the chosen version works throughout your APIs, schemas, clients, and database releases.
- Benchmark before changing a live system. Test representative insert and query patterns on the target engine. The standard explains why locality can differ, but it does not establish the performance gain for a particular deployment.
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