MySQL Select 不在不同的 Where 值上使用索引

Question

我有一个有时完美运行的连接查询 我从 EventKeyReference 表中获取 eventKey、label 和 dataUnit，从 EventData 中获取 valueParam，从表 EventHeader 中获取时间戳。 在我的 where 子句中，我有 Unit u.id = 3。这很好用。但是，当我尝试搜索 Unit u.id = 1 时，查询有时可能需要长达 20 秒。

搜索 id = 3

SELECT ek.eventKey, ek.label, ek.dataUnit, ed.valueParam, eh.timestamp
FROM EventKeyReference AS ek 
INNER JOIN UnitReference_EventKeyReference AS urek ON urek.eventKeyReferences_id = ek.id 
INNER JOIN UnitReference AS ur ON ur.id = urek.UnitReference_id
INNER JOIN Unit AS un ON un.id = ur.unit_id INNER JOIN User AS u ON u.id = ur.user_id
INNER JOIN EventData AS ed ON ek.eventKey = ed.keyParam
INNER JOIN EventHeader AS eh ON eh.id = ed.eventHeader_id
WHERE ek.eventKey = 'C004' AND u.id = 3 AND un.serialNumber=221148096
ORDER BY eh.timestamp LIMIT 1;

给出这个:

+----------+----------+----------+------------+---------------------+
| eventKey | label    | dataUnit | valueParam | timestamp           |
+----------+----------+----------+------------+---------------------+
| C004     | Utgang 2 | on/off   | 0          | 2000-01-01 00:01:04 |
+----------+----------+----------+------------+---------------------+
1 row in set (0.00 sec)

搜索 id = 1 得到这个:

+----------+-------------+----------+------------+---------------------+
| eventKey | label       | dataUnit | valueParam | timestamp           |
+----------+-------------+----------+------------+---------------------+
| C004     | DigitalOut1 | ON/OFF   | 0          | 2000-01-01 00:01:04 |
+----------+-------------+----------+------------+---------------------+
1 row in set (16.04 sec)

如您所见，执行查询需要 16 秒。

在检查 EXPLAIN SELECT 时，我明白了。

    mysql> EXPLAIN SELECT ek.eventKey, ek.label, ek.dataUnit, ed.valueParam,    eh.timestamp   FROM EventKeyReference AS ek INNER JOIN UnitReference_EventKeyReference AS urek ON urek.eventKeyReferences_id = ek.id INNER JOIN UnitReference AS ur ON ur.id = urek.UnitReference_id  INNER JOIN Unit AS un ON un.id = ur.unit_id INNER JOIN User AS u ON u.id = ur.user_id INNER JOIN EventData AS ed ON ek.eventKey = ed.keyParam INNER JOIN EventHeader AS eh ON eh.id = ed.eventHeader_id   WHERE ek.eventKey = 'C004' AND u.id = 1 AND un.serialNumber=221148096 ORDER BY eh.timestamp DESC LIMIT 1;
+----+-------------+-------+--------+-----------------------------------------------+--------------------+---------+------------------------------------+--------+----------------------------------------------+
| id | select_type | table | type   | possible_keys                                 | key                | key_len | ref                                | rows   | Extra                                        |
+----+-------------+-------+--------+-----------------------------------------------+--------------------+---------+------------------------------------+--------+----------------------------------------------+
|  1 | SIMPLE      | u     | const  | PRIMARY                                       | PRIMARY            | 8       | const                              |      1 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | eh    | index  | PRIMARY                                       | eh_timestamp       | 9       | NULL                               | 169161 | Using index                                  |
|  1 | SIMPLE      | ed    | ref    | FK794A194417A622D6                            | FK794A194417A622D6 | 9       | fltcoor.eh.id                      |      5 | Using where                                  |
|  1 | SIMPLE      | ur    | range  | PRIMARY,FK83382A0747140EFE,FK83382A073EC5085E | FK83382A0747140EFE | 9       | NULL                               |      3 | Using where; Using join buffer               |
|  1 | SIMPLE      | urek  | ref    | PRIMARY,FKAC98AB0E2DFCBD8F,FKAC98AB0E564AEB96 | FKAC98AB0E564AEB96 | 8       | fltcoor.ur.id                      |     11 | Using index                                  |
|  1 | SIMPLE      | un    | eq_ref | PRIMARY,serialNumber                          | PRIMARY            | 8       | fltcoor.ur.unit_id                 |      1 | Using where                                  |
|  1 | SIMPLE      | ek    | eq_ref | PRIMARY                                       | PRIMARY            | 8       | fltcoor.urek.eventKeyReferences_id |      1 | Using where                                  |
+----+-------------+-------+--------+-----------------------------------------------+--------------------+---------+------------------------------------+--------+----------------------------------------------+
7 rows in set (0.00 sec)

mysql> EXPLAIN SELECT ek.eventKey, ek.label, ek.dataUnit, ed.valueParam, eh.timestamp   FROM EventKeyReference AS ek INNER JOIN UnitReference_EventKeyReference AS urek ON urek.eventKeyReferences_id = ek.id INNER JOIN UnitReference AS ur ON ur.id = urek.UnitReference_id  INNER JOIN Unit AS un ON un.id = ur.unit_id INNER JOIN User AS u ON u.id = ur.user_id INNER JOIN EventData AS ed ON ek.eventKey = ed.keyParam INNER JOIN EventHeader AS eh ON eh.id = ed.eventHeader_id   WHERE ek.eventKey = 'C004' AND u.id = 3 AND un.serialNumber=221148096 ORDER BY eh.timestamp DESC LIMIT 1;
+----+-------------+-------+--------+-----------------------------------------------+--------------------+---------+------------------------------------+------+-------------+
| id | select_type | table | type   | possible_keys                                 | key                | key_len | ref                                | rows | Extra       |
+----+-------------+-------+--------+-----------------------------------------------+--------------------+---------+------------------------------------+------+-------------+
|  1 | SIMPLE      | u     | const  | PRIMARY                                       | PRIMARY            | 8       | const                              |    1 | Using index |
|  1 | SIMPLE      | eh    | index  | PRIMARY                                       | eh_timestamp       | 9       | NULL                               |    1 | Using index |
|  1 | SIMPLE      | ur    | ref    | PRIMARY,FK83382A0747140EFE,FK83382A073EC5085E | FK83382A0747140EFE | 9       | const                              |    2 | Using where |
|  1 | SIMPLE      | un    | eq_ref | PRIMARY,serialNumber                          | PRIMARY            | 8       | fltcoor.ur.unit_id                 |    1 | Using where |
|  1 | SIMPLE      | ed    | ref    | FK794A194417A622D6                            | FK794A194417A622D6 | 9       | fltcoor.eh.id                      |    5 | Using where |
|  1 | SIMPLE      | urek  | ref    | PRIMARY,FKAC98AB0E2DFCBD8F,FKAC98AB0E564AEB96 | FKAC98AB0E564AEB96 | 8       | fltcoor.ur.id                      |   11 | Using index |
|  1 | SIMPLE      | ek    | eq_ref | PRIMARY                                       | PRIMARY            | 8       | fltcoor.urek.eventKeyReferences_id |    1 | Using where |
+----+-------------+-------+--------+-----------------------------------------------+--------------------+---------+------------------------------------+------+-------------+
7 rows in set (0.00 sec)

出于某种原因，在 id = 1 的查询中，MYSQL 会处理大约 16.000 行。

这一直困扰着我一整天，因为我找不到为什么会发生这种情况。如果有任何更多信息，我很乐意提供，因为我自己远非 Mysql 专家。

Answer 1

正如 Jakob 所说，如果您有 160000 个用户 1 的条目，那可能是您的瓶颈。我已经重写希望能有所帮助。我更改了顺序，希望将最小的粒度部分放在查询的主要位置并从中链接起来。我怀疑给定序列号的记录很少，因此将其放入 WHERE 子句并添加次要的 JOIN 条件(通过实现的 ON/AND 子句)应该会有所帮助。

SELECT 
      ek.eventKey, 
      ek.label, 
      ek.dataUnit, 
      ed.valueParam, 
      eh.timestamp
   FROM 
      Unit AS un 
         INNER JOIN UnitReference AS ur 
            ON un.id = ur.unit_id
            AND ur.user_id = 3      <==  ADDED USER ID CRITERIA HERE
            INNER JOIN User AS u 
               ON ur.user_id = u.id
            INNER JOIN UnitReference_EventKeyReference AS urek 
               ON ur.id = urek.UnitReference_id
               INNER JOIN EventKeyReference AS ek 
                  ON urek.eventKeyReferences_id = ek.id
                  AND ek.eventKey = 'C004'   <== ADDED EVENT KEY CRITERIA HERE
               INNER JOIN EventData AS ed 
                  ON ek.eventKey = ed.keyParam
                  INNER JOIN EventHeader AS eh 
                     ON ed.eventHeader_id = eh.id
   WHERE 
      un.serialNumber=221148096
   ORDER BY 
      eh.timestamp LIMIT 1;

如果上述方法不起作用，我会尝试的唯一其他项目，并且因为它是 MySQL，我会再添加一个关键字...

SELECT STRAIGHT_JOIN [查询的其余部分]

它告诉 MySQL 按照您指定的顺序进行查询，而不是尝试以不同的顺序为您考虑。