是否可以使用EXISTS而不是DISTINCT的IN子句来优化查询 [英] Is it possible to optimize query using the EXISTS instead of IN clause with DISTINCT

问题描述

我有一个有效的查询。

select contract_no AS c_no, cm_mac AS c_mc, MIN(tstamp) as time2, sum(1) as aps
from devices where 
contract_no in 
(select distinct(contract_no) from devices where 
tstamp >= '2018-10-28 06:59:59' AND tstamp <= '2018-10-29 07:00:00')
group by contract_no, cm_mac;

我意识到查询速度很慢，所以我想知道是否仍然可以优化该查询？
我当时在考虑也许使用EXISTS而不是IN，但是在那种情况下，我不能以 EXISTS（从....中选择SELECT 1，其中contract_no = contract_no）开始我需要这个 DISTINCT 子句。

I realized that query is slow so I was wondering if it is possible in anyway to optimize this query? I was thinking maybe to use EXISTS instead of IN but in that case I cannot start with EXISTS (SELECT 1 from .... where contract_no= contract_no ) because I need this DISTINCT clause.

当然我需要返回相同的结果。
是否可以通过某种方式优化此查询？

Of course I need to return same results. Is this possible to somehow optimize this query?

UPDATE:

我检查了反馈，您说得对。如果执行这两个查询，我将得到相同的结果。
但关键是完整查询更为复杂，如果我没有此子查询，则会得到更多结果。

I checked feedbacks and you are right. If those two queries are executed I am getting the same results. But the point is that the full query is more complex, and if I do not have this sub query I am getting more results.

查询1（返回72行是正确的）：

QUERY 1 (Returns 72 rows which is Correct):

    SELECT id, contract_no, customer, address, cm_mac, aps 
    FROM (select * from new_installed_devices where  insert4date >='2018-10-28' 
    AND insert4date <='2018-10-28' AND install_mark<2) as d1 
left join 
( select * from (select contract_no AS c_no, cm_mac AS c_mc, 
MIN(tstamp) as time2, sum(1) as aps from devices_change 
where contract_no in (select distinct(contract_no) from devices_change 
where tstamp >= '2018-10-28 06:59:59' AND tstamp <= '2018-10-29 07:00:00') 
group by contract_no, cm_mac ) as mtmbl 
where mtmbl.time2 >= '2018-10-28 06:59:59' and mtmbl.time2 <= '2018-10-29 
07:00:00' ) as tmp on d1.contract_no=tmp.c_no 
where aps>0 group by contract_no, customer, address, cm_mac;

查询2（返回不正确的75行），此方法有您的建议（包括两个内的查询）：

QUERY 2 (Returns 75 rows which is not Correct) and this approach has your recommendation (to include two queries within the one):

SELECT id, contract_no, customer, address, cm_mac, aps  
FROM (select * from new_installed_devices where  insert4date >='2018-10-28' 
AND insert4date <='2018-10-28' AND install_mark<2) as d1 left join 
( select * from (select distinct(contract_no) AS c_no, cm_mac AS c_mc, 
MIN(tstamp) as time2, sum(1) as aps from devices_change 
where  tstamp >= '2018-10-28 06:59:59' AND tstamp <= '2018-10-29 07:00:00'
 group by contract_no, cm_mac ) as mtmbl 
where mtmbl.time2 >= '2018-10-28 06:59:59' and 
mtmbl.time2 <= '2018-10-29 07:00:00' ) as tmp 
on d1.contract_no=tmp.c_no 
where aps>0 group by contract_no, customer, address, cm_mac;

推荐答案

尝试此版本：

select contract_no AS c_no, cm_mac AS c_mc, min(tstamp) as time2, count(*) as aps
from devices d
where exists (select 1
              from devices d2
              where d2.contract_no = d.contract_no and
                    tstamp >= '2018-10-28 06:59:59' and
                    tstamp <= '2018-10-29 07:00:00'
              )
group by contract_no, cm_mac;

您要在设备（合同编号，tstamp）上建立索引。

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