UUID version 7

What a v7 is made of#

RFC 9562 § 5.7 · UUID Version 7The layout, field by field

Why it sorts and v4 does not#

A B-tree index writes new rows next to the keys they sort near. Random keys therefore scatter writes across the whole index, dirtying pages everywhere and pushing the working set out of memory. A v7 puts the clock in the high bits, so consecutive inserts land on the same page, and the index behaves like one built on an auto-increment column while keeping the properties that made you choose a UUID in the first place.

RFC 9562 § 6.11 · SortingSorting, and what the standard expects of a time-ordered layout

v7 against v4 and ULID#

A ULID and a v7 hold the same clock and differ mainly in how they print. If the identifiers have to live in a uuid column or travel through a library that validates the version nibble, v7 is the one that fits without conversion.

What you give up#

A v7 tells anyone holding it roughly when it was created. For an order ID that is usually harmless; for a password reset token, an invitation link or anything a competitor might count, it leaks. It also has 48 fewer random bits than a v4, which is irrelevant for collisions and relevant for guessability: a v7 is not a secret and should never be used as one.

RFC 9562 § 6.9 · UnguessabilityUnguessability: why an identifier is not a capability

Two v7s in the same millisecond#

Within a single millisecond the clock field is identical, so ordering falls to whatever the generator put in rand_a. Implementations that care spend those twelve bits on a counter that increments while the millisecond lasts; implementations that do not fill them with randomness, and identifiers from the same millisecond come back in arbitrary order. If per-millisecond ordering matters to you, check which one your library does.

RFC 9562 § 6.2 · Monotonicity and CountersThe counter methods the standard describes

-- PostgreSQL 18 ships a generator
CREATE TABLE orders (
    id uuid PRIMARY KEY DEFAULT uuidv7(),
    created_at timestamptz NOT NULL DEFAULT now()
);

-- keep the column anyway: the identifier is an ordering hint, not an audit trail