Comment gérer plusieurs jointures

Réécriture en UNION c'est simple, copiez le source et supprimez l'un des OR ed conditions dans chacun :

SELECT 
   pending_corrections.corrected_plate , pending_corrections.seenDate
FROM
    (pending_corrections
    INNER JOIN cameras ON pending_corrections.camerauid = cameras.camera_id)
        INNER JOIN
    vehicle_vrn ON (pending_corrections.corrected_plate = vehicle_vrn.vrn500)
        INNER JOIN
    vehicle_ownership ON vehicle_vrn.fk_sysno = vehicle_ownership.fk_sysno
WHERE
    pending_corrections.seenDate >= '2015-01-01 00:00:00'
        AND pending_corrections.seenDate <= '2015-01-31 23:59:59'

union 

SELECT 
   pending_corrections.corrected_plate , pending_corrections.seenDate
FROM
    (pending_corrections
    INNER JOIN cameras ON pending_corrections.camerauid = cameras.camera_id)
        INNER JOIN
    vehicle_vrn ON pending_corrections.corrected_plate = vehicle_vrn.vrnno)
        INNER JOIN
    vehicle_ownership ON vehicle_vrn.fk_sysno = vehicle_ownership.fk_sysno
WHERE
    pending_corrections.seenDate >= '2015-01-01 00:00:00'
        AND pending_corrections.seenDate <= '2015-01-31 23:59:59'

ORDER BY 1,2;

Existe-t-il un index sur pending_corrections.seenDate ?