问sql优化 ，百万联表后查 count(*)时间过长？

explain SELECT count(*)
FROM
    pension_xunying_record xr,
    pension_hme_and_user hau,
    pension_medical_device md,
    pension_user pu 
WHERE
    xr.device_id = hau.id 
    AND hau.md_id = md.id 
    AND hau.user_id = pu.user_id 
    AND hau.is_unbind = 0 
    AND xr.extend = 0 
ORDER BY
    xr.id DESC

select count(1)
from  (select * from pension_xunying_record as xr where xr.extend=0) as xr1
inner join (select * from pension_hme_and_user as hau where hau.is_unbind=0 ) as hau1
on xr1.device_id = hau1.id 
inner join (select *    from     pension_user as pu ) as pu1
on pu1.user_id =hau1.user_id 
inner join (select    * from pension_medical_device as md) as md1
on md1.id = hau1.md_id