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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11For a new database key, UUIDv7 is usually the most direct choice when time ordering helps your index and exposing approximate creation time is acceptable. It is a standards-based 128-bit UUID designed to sort by generation time. UUIDv4 remains appropriate when random-looking identifiers matter more than insertion locality; ULID is a time-sortable alternative when its text representation and library support fit your stack. None guarantees a fixed performance gain: test against your database and workload before changing existing keys.
How UUIDv4, UUIDv7, and ULID differ
All three formats can identify records with 128-bit values, but they encode and order those values differently. That distinction affects insertion patterns and what an identifier reveals.
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| Comparison | UUIDv4 | UUIDv7 | ULID |
|---|---|---|---|
| What it encodes | Random or pseudorandom value; no generation-time order. RFC 9562 | Unix-millisecond timestamp in the most significant 48 bits; the remaining 74 non-version/non-variant bits can use random data or optional precision and monotonicity mechanisms. RFC 9562 | 48-bit Unix-millisecond timestamp plus 80 bits of randomness. ULID specification |
| Ordering and index locality | Generated values are scattered across keyspace, so inserts by key can land at distant index positions. RFC 9562 | Designed to sort as opaque bytes by time, placing nearby generation times near each other in the ordered keyspace. RFC 9562 | Canonical strings sort by time under the specified character ordering. Within one millisecond, order depends on the generator. ULID specification |
| Canonical text form | 36-character UUID string; binary storage is also possible. RFC 9562 | 36-character UUID string; binary storage is also possible. RFC 9562 | 26 Crockford Base32 characters. The shorter text form alone does not prove smaller database storage. ULID specification |
| Timestamp visibility | No embedded timestamp. | Reveals approximate creation time and relative order. | Reveals approximate creation time and relative order. |
| Standards and database support | RFC UUID version; support depends on the database and generator. | Standardized by RFC 9562; PostgreSQL 18 documents native UUIDv7 generation and UUID storage for any version. | Separate specification and ecosystem convention; check the database representation, comparison rules, and library behavior. |
For UUIDs, RFC 9562 recommends storing the underlying 128-bit binary value where feasible. It notes that text requires 288 bits to represent a UUID, while binary storage can take less space and may improve data access. This storage guidance is about UUIDs; do not infer that a 26-character ULID string will be smaller or faster in a particular database. The column type and index representation determine the actual footprint.
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A B-tree index organizes keys in sorted order. With UUIDv4, new random values can require inserts at unrelated positions. RFC 9562 describes this as poor database-index locality and says it can have a dramatic negative effect on B-trees and variants. UUIDv7 and time-ordered ULIDs instead put new values near the recent end of the ordered keyspace, which is a locality advantage by design. The RFC summarizes the principle: “Time-ordered monotonic UUIDs benefit from greater database-index locality because the new values are near each other in the index.”
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Locality is not a guarantee that an index will never fragment or that a workload will become faster by a particular percentage. The available primary sources do not establish a controlled, generally applicable comparison of page splits, index size, write amplification, cache hit rate, latency, or read performance for these three identifiers. Engine behavior, schema, concurrency, generator implementation, and query patterns all affect the result.
Benchmark before treating a format change as an optimization
Compare formats using the same schema, row shape, indexes, database version, hardware, concurrency, transaction settings, and workload. Include realistic insert bursts, multiple generator instances, and clock behavior. Measure:
- Insert throughput and latency.
- Index size and engine-specific page-split or index-health indicators.
- Index maintenance behavior and WAL or log volume, where relevant.
- Representative read and query latency, not only inserts.
These are measurements to run on your target system, not published results for a universal workload.
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Generation order, monotonicity, and privacy
Same-millisecond and multi-generator behavior
UUIDv7 timestamps have millisecond resolution. RFC 9562 allows optional sub-millisecond precision and monotonicity mechanisms for generating multiple UUIDs during one timestamp tick, but the guarantee depends on the generator. PostgreSQL 18 documents its uuidv7() function as using Unix timestamp milliseconds plus sub-millisecond timestamp and random data.
The ULID specification describes a monotonic factory that increments the random component when it detects multiple calls in the same millisecond. Do not assume every ULID library uses that strategy. Verify the actual generator’s process or node scope, behavior when the clock moves backward, and handling of overflow. Without a monotonic strategy, same-millisecond ordering is not guaranteed by the ULID specification.
Time-bearing identifiers are not secrets
UUIDv7 and ULID reveal approximate creation time and ordering. RFC 9562 notes that UUID timestamps create a small attack surface and recommends UUIDv4 when UUIDs are required for a security operation in an application context. Regardless of version, an identifier should not serve as an authorization secret: enforce access control independently of whether a value is difficult to guess.
PostgreSQL 18: native UUIDv7 support
PostgreSQL 18 documents uuid as a 128-bit type that stores UUIDs conforming to RFC 9562 and accepts values from any UUID version, regardless of origin. It provides native UUIDv4 and UUIDv7 generation. That means a UUID-typed column can hold newly generated UUIDv7 values; it does not, by itself, convert existing keys or update foreign-key references.
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PostgreSQL 18’s UUID functions documentation describes uuidv7() as time ordered and includes version and timestamp extraction functions. uuid_extract_timestamp works for versions 1 and 7, but PostgreSQL cautions that the extracted time is not necessarily the exact generation time because it depends on the generating implementation. Confirm the server version before relying on these functions. See the PostgreSQL UUID type documentation for storage and accepted values.
Which identifier should you choose?
Choose UUIDv7 for new records when locality matters
UUIDv7 is a practical default for a new system if your stack has a maintained, correct generator, time-based ordering is useful for the target workload, and approximate creation-time exposure is acceptable. It preserves the UUID format while providing ordered values, and PostgreSQL 18 can generate it natively.
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Keep or choose UUIDv4 when randomness is valuable
UUIDv4 is still reasonable when random-looking identifiers suit your application or when the benefits of time ordering do not outweigh timestamp visibility and the other tradeoffs. Its index locality may be worse for insert-heavy B-tree workloads, so use measured index behavior—not the format alone—to decide whether that matters in your system.
Choose ULID when the representation and ecosystem fit
ULID can be a good fit when its 26-character Crockford Base32 form and available libraries are meaningful advantages. Check whether the database stores it as text or binary, how values are compared, and whether all generators provide the within-millisecond behavior your application expects. Its compact text form is a presentation characteristic, not evidence by itself of reduced database storage or improved index performance.
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Changing identifiers is a measured optimization, not routine cleanup. First establish whether random-key insertion is a meaningful cost by profiling index size, insert behavior, write load, cache pressure, and query performance. If another bottleneck dominates, changing IDs may not address it.
If measurements support a migration, map every place the identifier is stored or relied upon before choosing a rollout. Primary keys often appear in foreign keys, external APIs, event payloads, caches, replicas, and application assumptions. A migration plan should address compatibility during rollout, uniqueness validation, backfill sequencing, rollback, and cutover. The RFC and PostgreSQL documentation do not prescribe a universal zero-downtime procedure or a threshold at which migration is worthwhile.
A lower-risk option: use UUIDv7 only for new rows
Keeping existing UUIDv4 values while generating UUIDv7 for new records can avoid rekeying legacy data. This works only if every consumer accepts mixed UUID versions and the application does not assume that every key is time ordered. Validate the mixed population against your workload and integrations; support for multiple UUID versions does not determine how an application handles them.
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