Original query:

SELECT
    *
FROM
    appointments_customers
WHERE
    appointment_id IN (
        SELECT
            id
        FROM
            "appointments"
        WHERE
            "agenda_id" IN (3943, 5699, 1525, 4006, 3221, 2858, 1528, 1514, 4967, 5698, 5452, 5969, 2147, 1512, 5453)
            AND ("start_at" <= '2019-01-14T00:00:00.000+00:00'
                AND "end_at" >= '2019-01-01T00:00:00.000+00:00'));