Postgres PL/pgSQL function for UUID v7 and a bonus custom UUID v8 to support microsecond precision as well. Read more here: https://datatracker.ietf.org/doc/draft-peabody-dispatch-new-uuid-format/

A_UUID_v7_for_Postgres.sql

-- Based off IETF draft, https://datatracker.ietf.org/doc/draft-peabody-dispatch-new-uuid-format/

create or replace function uuid_generate_v7()
returns uuid
as $$
begin
  -- use random v4 uuid as starting point (which has the same variant we need)
  -- then overlay timestamp
  -- then set version 7 by flipping the 2 and 1 bit in the version 4 string
  return encode(
    set_bit(
      set_bit(
        overlay(uuid_send(gen_random_uuid())
                placing substring(int8send(floor(extract(epoch from clock_timestamp()) * 1000)::bigint) from 3)
                from 1 for 6
        ),
        52, 1
      ),
      53, 1
    ),
    'hex')::uuid;
end
$$
language plpgsql
volatile;

-- Generate a custom UUID v8 with microsecond precision
create or replace function uuid_generate_v8()
returns uuid
as $$
declare
  timestamp    timestamptz;
  microseconds int;
begin
  timestamp    = clock_timestamp();
  microseconds = (cast(extract(microseconds from timestamp)::int - (floor(extract(milliseconds from timestamp))::int * 1000) as double precision) * 4.096)::int;

  -- use random v4 uuid as starting point (which has the same variant we need)
  -- then overlay timestamp
  -- then set version 8 and add microseconds
  return encode(
    set_byte(
      set_byte(
        overlay(uuid_send(gen_random_uuid())
                placing substring(int8send(floor(extract(epoch from timestamp) * 1000)::bigint) from 3)
                from 1 for 6
        ),
        6, (b'1000' || (microseconds >> 8)::bit(4))::bit(8)::int
      ),
      7, microseconds::bit(8)::int
    ),
    'hex')::uuid;
end
$$
language plpgsql
volatile;

CONTRIBUTORS.txt

PERFORMANCE: Move from pgcrypto to built-in gen_random_uuid():
  Curtis Summers (https://github.com/csummers)

PERFORMANCE: Use set_bit to upgrade v4 to v7, not set_byte:
PERFORMANCE: Reduce local variable use while still being maintainable
  Rolf Timmermans (https://github.com/rolftimmermans)

LICENSE.txt

Copyright 2023 Kyle Hubert <kjmph@users.noreply.github.com> (https://github.com/kjmph)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.