倆張表 orders members
根據(jù)where條件刷選的數(shù)據(jù)大概是30幾萬
select count(*) from orders where order_status = 'active' and ship_status = '1'
【執(zhí)行的結(jié)果大概是100多毫秒】
select count（o.order_id） from orders o left join members m on o.member_id = m.member_id where o.order_status = 'active' and o.ship_status = '1'
【執(zhí)行的結(jié)果大概是1.6秒左右】

select count(*) from (select o.order_id from orders o left join members m on o.member_id = m.member_id where o.order_status = 'active' and o.ship_status = '1') tmp
【執(zhí)行的結(jié)果大概是1.6秒左右】

理論上來說執(zhí)行時間應(yīng)該差距不大的，sql的執(zhí)行計劃應(yīng)該也是最后才進(jìn)行count統(tǒng)計，統(tǒng)計為什么會有這么大的差距呢？求解