Warum verwendet MySQL bei JOIN plus ORDER nicht den Primärschlüssel?

Können Sie diese Version ausprobieren (sie ruft grundsätzlich zuerst die 420 Zeilen des corder Tabelle, behält die 20 von ihnen und führt dann die 3 äußeren Joins aus):

SELECT
    corder.id,
    corder.public_id,
    CONCAT(buyer.fname," ",buyer.lname) AS buyer_name,
    corder.status,
    corder.payment,
    corder.reserved AS R,
    corder.tracking_id != "" AS A,
    corder.payment_received AS pay_date,
    invoice.invoice_no AS inv,
    invoice.receipt_no AS rec,
    invoice.public AS pub_inv,
    proforma.proforma_no AS prof,
    proforma.public AS pub_pf,
    corder.rating,
    corder.rating_comments!="" AS got_comment
FROM
    ( SELECT * 
      FROM corder
      ORDER BY
        id DESC 
      LIMIT 400, 20
    )
    AS corder
LEFT JOIN user as buyer ON buyer.id = corder.buyer
LEFT JOIN invoice AS invoice ON invoice.id = corder.invoice
LEFT JOIN invoice AS proforma ON proforma.id = corder.proforma
ORDER BY
    corder.id DESC ;