create table t1 (id uuid, data text, primary key (id));

WITH upsert AS
    (UPDATE
        t1
    SET
        data = 'test'
    WHERE
        id='ad400a94-ad7a-4375-92c6-7294e3e4ce6d'
    RETURNING
        id)
INSERT INTO
    t1 (id,
    data)
SELECT
    'ad400a94-ad7a-4375-92c6-7294e3e4ce6d',
    'test'
WHERE
    NOT EXISTS
        (SELECT
            true
        FROM
            upsert);

BEGIN;

WITH upsert AS
    (UPDATE
        t1
    SET
        data = 'test'
    WHERE
        id='ad400a94-ad7a-4375-92c6-7294e3e4ce6d'
    RETURNING
        id)
INSERT INTO
    t1 (id,
    data)
SELECT
    'ad400a94-ad7a-4375-92c6-7294e3e4ce6d',
    'test'
WHERE
    NOT EXISTS
        (SELECT
            true
        FROM
            upsert);

-- In separate session

WITH upsert AS
    (UPDATE
        t1
    SET
        data = 'test'
    WHERE
        id='ad400a94-ad7a-4375-92c6-7294e3e4ce6d'
    RETURNING
        id)
INSERT INTO
    t1 (id,
    data)
SELECT
    'ad400a94-ad7a-4375-92c6-7294e3e4ce6d',
    'test'
WHERE
    NOT EXISTS
        (SELECT
            true
        FROM
            upsert);

-- In original session

END;