MySQL：子查询优化问题，其中子查询检查超过14000行

问题描述

我需要帮助来优化下面的子查询。简而言之，我有以下查询，其中根据子查询条件，tree表联接了branch上的s_id表和timestamp表的最大branch。>

我对该查询返回的结果感到满意。但是，此查询非常慢。瓶颈是从属子查询（branch2），它检查14000多行。如何优化子查询以加快查询速度？

SELECT *                
FROM dept.tree tree              
    LEFT JOIN dept.branch branch ON tree.s_id = branch.s_id                    
        AND branch.timestamp =
            (
                SELECT MAX(timestamp)                                   
                FROM dept.branch branch2 
                WHERE branch2.s_id = tree.s_id 
                AND branch2.timestamp <= tree.timestamp
            )                
WHERE tree.timestamp BETWEEN CONVERT_TZ('2020-05-16 00:00:00','America/Toronto','UTC') 
AND CONVERT_TZ('2020-05-16 23:59:59','UTC') 
AND tree.s_id IN ('459','460')                
ORDER BY tree.timestamp ASC;

表树：

   id     Box_id        timestamp       
373001645   1       2020-05-07 06:00:20 
373001695   1       2020-05-07 06:02:26
373001762   1       2020-05-07 06:05:17
373001794   1       2020-05-07 06:06:38
373001810   2       2020-05-07 06:07:21

表分支：

   id     Box_id        timestamp                  data
373001345   1       2020-05-07 06:00:20     {"R": 0.114,"H": 20.808}
373001395   1       2020-05-07 06:02:26     {"R": 0.12,"H": 15.544}
373001462   1       2020-05-07 06:03:01     {"R": 0.006,"H": 55.469}
373001494   1       2020-05-07 06:04:38     {"R": 0.004,"H": 51.85}
373001496   1       2020-05-07 06:05:18     {"R": 0.02,"H": 5.8965}
373001497   1       2020-05-07 06:06:39     {"R": 0.12,"H": 54.32}
373001510   2       2020-05-07 06:07:09     {"R": 0.34,"H": 1.32}
373001511   2       2020-05-07 06:07:29     {"R": 0.56,"H": 32.7}

分支具有s_id和时间戳索引

我正在使用5.7.25-google-log版本

EXPLAIN提供以下内容：

    id    select_type        table     partitions    type    possible_keys    key    key_len    ref    rows    filtered     Extra
    1   PRIMARY tree        range   unique_timestamp_s_id,idx_s_id_timestamp,idx_timestamp  idx_s_id_timestamp  10      2629    100.00  Using index condition; Using filesort
    1   PRIMARY branch      ref unique_timestamp_s_id,idx_timestamp unique_timestamp_s_id   5   func    1   100.00  Using where
    2   DEPENDENT SUBQUERY  branch2     ref unique_timestamp_s_id,idx_timestamp  idx_s_id_timestamp  5   tree.s_id   14122   33.33   Using where; Using index

解决方法

请提供SHOW CREATE TABLE。

branch需要INDEX(s_id,timestamp)

您需要LEFT吗？可能无缘无故拖慢了查询速度。

一列上的IN和另一上的BETWEEN的组合可能优化不佳；您正在运行什么版本？

请提供EXPLAIN SELECT，以便我们讨论其是否经过优化。如果不是，我们可以讨论如何将IN（OR的变体）转换为UNION。

这实际上可能比我上面想的要快...

具有上面的索引，然后大幅重写查询：

SELECT b.*
    FROM ( SELECT  s_id,MAX(timestamp) as timestamp                                
               FROM dept.branch 
               WHERE timestamp BETWEEN
                      CONVERT_TZ('2020-05-16 00:00:00','America/Toronto','UTC')
                  AND CONVERT_TZ('2020-05-16 23:59:59','UTC')
                 AND s_id IN ('459','460')                
         ) AS x
    JOIN dept.branch AS b  USING(s_id,timestamp)

首先，查看是否获得正确的信息。然后，我将解释如何在子查询中执行UNION（如果需要帮助）。

这应该更快：

select
    tree.s_id,tree.timestamp,branch.data
from
    (
        SELECT
            tree.s_id,max(branch.timestamp) as max_branch_timestamp
        FROM
            dept.tree tree
                LEFT JOIN dept.branch branch
                ON(
                        branch.s_id      =  tree.s_id
                    and branch.timestamp <= tree.timestamp
                )
        WHERE
            tree.timestamp BETWEEN
                CONVERT_TZ('2020-05-16 00:00:00','UTC') AND
                CONVERT_TZ('2020-05-16 23:59:59','UTC')
        AND tree.s_id IN ('459','460')                
        group by tree.s_id,tree.timestamp
    ) tree
        left outer join branch
        on(
                branch.s_id      = tree.s_id
            and branch.timestamp = tree.max_branch_timestamp
        )

groupwise-maximum mysql query-optimization query-performance subquery