MySQL group by 会影响查询性能

Question

我有 MySQL 查询，当前选择并加入 13 个表，最后对约 60k 行进行分组。没有分组的查询需要约 0 毫秒，但分组查询时间增加到约 1.7 秒。用于分组的字段是主字段并被索引。问题可能出在哪里？

我知道没有聚合的 group by 被认为是无效查询和不好的做法，但我需要不同的基表行并且不能使用 DISTINCT 语法。

查询本身如下所示：

SELECT `table_a`.*
FROM   `table_a` 
       LEFT JOIN `table_b` 
              ON `table_b`.`invoice` = `table_a`.`id` 
       LEFT JOIN `table_c` AS `r1` 
              ON `r1`.`invoice_1` = `table_a`.`id` 
       LEFT JOIN `table_c` AS `r2` 
              ON `r2`.`invoice_2` = `table_a`.`id` 
       LEFT JOIN `table_a` AS `i1` 
              ON `i1`.`id` = `r1`.`invoice_2` 
       LEFT JOIN `table_a` AS `i2` 
              ON `i2`.`id` = `r2`.`invoice_1` 
       JOIN `table_d` AS `_u0` 
         ON `_u0`.`id` = 1 
       LEFT JOIN `table_e` AS `_ug0` 
              ON `_ug0`.`user` = `_u0`.`id` 
       JOIN `table_f` AS `_p0` 
         ON ( `_p0`.`enabled` = 1 
              AND ( ( `_p0`.`role` < 2 
                      AND `_p0`.`who` IS NULL ) 
                     OR ( `_p0`.`role` = 2 
                          AND ( `_p0`.`who` = '0' 
                                 OR `_p0`.`who` = `_u0`.`id` ) ) 
                     OR ( `_p0`.`role` = 3 
                          AND ( `_p0`.`who` = '0' 
                                 OR `_p0`.`who` = `_ug0`.`group` ) ) ) ) 
            AND ( `_p0`.`action` = '*' 
                   OR `_p0`.`action` = 'read' ) 
            AND ( `_p0`.`related_table` = '*' 
                   OR `_p0`.`related_table` = 'table_name' ) 
       JOIN `table_a` AS `_e0` 
         ON ( ( `_p0`.`related_id` = 0 
                 OR `_p0`.`related_id` = `_e0`.`id` 
                 OR `_p0`.`related_user` = `_e0`.`user` 
                 OR `_p0`.`related_group` = `_e0`.`group` ) 
               OR ( `_p0`.`role` = 0 
                    AND `_e0`.`user` = `_u0`.`id` ) 
               OR ( `_p0`.`role` = 1 
                    AND `_e0`.`group` = `_ug0`.`group` ) ) 
            AND `_e0`.`id` = `table_a`.`id` 
       JOIN `table_d` AS `_u1` 
         ON `_u1`.`id` = 1 
       LEFT JOIN `table_e` AS `_ug1` 
              ON `_ug1`.`user` = `_u1`.`id` 
       JOIN `table_f` AS `_p1` 
         ON ( `_p1`.`enabled` = 1 
              AND ( ( `_p1`.`role` < 2 
                      AND `_p1`.`who` IS NULL ) 
                     OR ( `_p1`.`role` = 2 
                          AND ( `_p1`.`who` = '0' 
                                 OR `_p1`.`who` = `_u1`.`id` ) ) 
                     OR ( `_p1`.`role` = 3 
                          AND ( `_p1`.`who` = '0' 
                                 OR `_p1`.`who` = `_ug1`.`group` ) ) ) ) 
            AND ( `_p1`.`action` = '*' 
                   OR `_p1`.`action` = 'read' ) 
            AND ( `_p1`.`related_table` = '*' 
                   OR `_p1`.`related_table` = 'table_name' ) 
       JOIN `table_g` AS `_e1` 
         ON ( ( `_p1`.`related_id` = 0 
                 OR `_p1`.`related_id` = `_e1`.`id` 
                 OR `_p1`.`related_user` = `_e1`.`user` 
                 OR `_p1`.`related_group` = `_e1`.`group` ) 
               OR ( `_p1`.`role` = 0 
                    AND `_e1`.`user` = `_u1`.`id` ) 
               OR ( `_p1`.`role` = 1 
                    AND `_e1`.`group` = `_ug1`.`group` ) ) 
            AND `_e1`.`id` = `table_a`.`company` 
WHERE  `table_a`.`date_deleted` IS NULL 
       AND `table_a`.`company` = 4
       AND `table_a`.`type` = 1
       AND `table_a`.`date_composed` >= '2016-05-04 14:43:55' 
GROUP BY `table_a`.`id`

Answer 1

ORs 杀戮性能。

此复合索引可能会有所帮助：INDEX(company, type, date_deleted, date_composed)。

LEFT JOIN table_b ON table_b.invoice = table_a.id 似乎除了减慢处理速度之外什么也没做。未使用 table_b 或 SELECTed 的字段。由于它是一个LEFT 连接，它不会限制输出。等等。如果它摆脱它，或者证明它是合理的。

其他连接同上。

JOIN 和 GROUP BY 会发生什么：首先，执行所有连接；这会爆炸中间“表”中的行数。然后GROUP BY 内爆行集。

避免这种爆炸-内爆缓慢的一种技术是做

SELECT ...,
    ( SELECT ... ) AS ...,
    ...

而不是JOIN 或LEFT JOIN。但是，只有在子查询中有 0 行或 1 行时才有效。通常，当可以将聚合（例如 SUM）移动到子查询中时，这很有用。

如需进一步讨论，请添加SHOW CREATE TABLE。