sql优化-表容量大嵌套子查询

SELECT
    a.illumination 
FROM
    ( SELECT did, illumination, record_time FROM light_record WHERE DATEDIFF( record_time, NOW())= 0 ) AS a
    INNER JOIN device_new AS b ON ( a.did = b.Device_id AND b.Room_id = 1 ) 
ORDER BY
    a.record_time DESC 
    LIMIT 1;