如何处理多个连接

Question

我有一个复杂的查询，它需要总共 4 个表中的字段。内部联接导致查询花费的时间比应有的时间长得多。我有运行一个 EXPLAIN 语句，其可视化结果附在下面：

这是我的查询：

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
        OR 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 pending_corrections.corrected_plate , pending_corrections.seenDate ASC;

如何在其中一个连接中没有 OR 的情况下实现相同的效果？

Answer 1

您可以尝试以下方法：

select 
   pending_corrections.corrected_plate , pending_corrections.seenDate
from pending_corrections
where pending_corrections.seenDate >= '2015-01-01 00:00:00'
  and pending_corrections.seenDate <= '2015-01-31 23:59:59'
  and exists(select 1 from cameras where pending_corrections.camerauid = cameras.camera_id)
  and exists(select 1 from vehicle_ownership where vehicle_vrn.fk_sysno = vehicle_ownership.fk_sysno)
  and exists(select 1 from vehicle_vrn 
             where pending_corrections.corrected_plate in (vehicle_vrn.vrnno, vehicle_vrn.vrn500))
order by 1,2;

或者如 dnoeth 已经提到的那样：

select * from (
select 
   pending_corrections.corrected_plate , pending_corrections.seenDate
from pending_corrections
where pending_corrections.seenDate >= '2015-01-01 00:00:00'
  and pending_corrections.seenDate <= '2015-01-31 23:59:59'
  and exists(select 1 from cameras where pending_corrections.camerauid = cameras.camera_id)
  and exists(select 1 from vehicle_ownership where vehicle_vrn.fk_sysno = vehicle_ownership.fk_sysno)
  and exists(select 1 from vehicle_vrn where pending_corrections.corrected_plate = vehicle_vrn.vrnno)
union
select 
   pending_corrections.corrected_plate , pending_corrections.seenDate
from pending_corrections
where pending_corrections.seenDate >= '2015-01-01 00:00:00'
  and pending_corrections.seenDate <= '2015-01-31 23:59:59'
  and exists(select 1 from cameras where pending_corrections.camerauid = cameras.camera_id)
  and exists(select 1 from vehicle_ownership where vehicle_vrn.fk_sysno = vehicle_ownership.fk_sysno)
  and exists(select 1 from vehicle_vrn where pending_corrections.corrected_plate = vehicle_vrn.vrn500)
)  by 1,2;

PS 当然，如果没有数据和知道你所有的索引，我无法自己测试它

Answer 2

重写为 UNION 很简单，复制源代码并删除每个 ORed 条件之一：

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;

pending_corrections.seenDate 上有索引吗？

Answer 3

      ( SELECT  pc.corrected_plate , pc.seenDate
            FROM  pending_corrections AS pc
            INNER JOIN  cameras AS c ON pc.camerauid = c.camera_id
            INNER JOIN  vehicle_vrn AS v ON pc.corrected_plate = v.vrn500
            INNER JOIN  vehicle_ownership AS vo ON v.fk_sysno = vo.fk_sysno
            WHERE  pc.seenDate >= '2015-01-01'
              AND  pc.seenDate  < '2015-01-01' + INTERVAL 1 MONTH  -- note improved pattern
      )
    UNION  ALL   -- or use DISTINCT if you could have dups
      ( SELECT  pc.corrected_plate , pc.seenDate
            FROM  pending_corrections AS pc
            INNER JOIN  cameras AS c ON pc.camerauid = c.camera_id
            INNER JOIN  vehicle_vrn AS v ON pc.corrected_plate = v.vrnno
            INNER JOIN  vehicle_ownership AS vo ON v.fk_sysno = vo.fk_sysno
            WHERE  pc.seenDate >= '2015-01-01'
              AND  pc.seenDate  < '2015-01-01' + INTERVAL 1 MONTH 
      )
    ORDER BY  corrected_plate , seenDate;

你需要

pc: INDEX(seenDate)  -- which you said you have
c:  INDEX(camera_id) -- unless you have PRIMARY KEY(camera_id)
v:  INDEX(vrn500)
v:  INDEX(vrnno)
vo: INDEX(fk_sysno) -- sounds like it already exists

如何处理多个连接

How to handle multiple joins

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