Pourquoi MySQL n'utilise-t-il pas la clé primaire sur JOIN plus ORDER ?

Pouvez-vous essayer cette version (elle obtient essentiellement les 420 premières lignes du corder table, garde les 20 d'entre eux et fait ensuite les 3 jointures externes) :

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 ;