I dug into the internals of the local, SQLite version of this just now and wrote up some notes here: https://til.simonwillison.net/deno/deno-kv#user-content-deno...

The most interesting detail is probably the schema they're using for that:

    CREATE TABLE queue (
      ts integer not null,
      id text not null,
      data blob not null,
      backoff_schedule text not null,
      keys_if_undelivered blob not null,
      primary key (ts, id)
    );
    CREATE TABLE queue_running(
      deadline integer not null,
      id text not null,
      data blob not null,
      backoff_schedule text not null,
      keys_if_undelivered blob not null,
      primary key (deadline, id)
    );
    CREATE INDEX kv_expiration_ms_idx on kv (expiration_ms);