MySQL联表查询的索引使用

十毛

发布于 2019-03-27 14:58:00

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发布于 2019-03-27 14:58:00

文章被收录于专栏：用户1337634的专栏

项目中一般使用的都是单表查询，但是在一些业务场景下，偶尔会选择联表查询，一直对联表查询时如何使用索引一直感到很好奇。正好近期项目中遇到一个问题，联表查询时，没有建立索引，耗时居然达到了可耻的10分钟，所以趁机了解了一下。

表数据

一共3张表knowledge, knowledge_question, knowledge_answer，数据在6000~10000之间。

knowledge： 6126
knowledge_question：9647
knowledge_answer：8267

执行的语句：

SELECT DISTINCT(k.base_id) FROM knowledge AS k 
LEFT JOIN knowledge_question AS q ON k.id=q.knowledge_id 
LEFT JOIN knowledge_answer AS a ON k.id=a.knowledge_id 
WHERE k.update_time>'2019-01-01 12:00:00' AND q.update_time>'2019-01-01 12:00:00' AND a.update_time>'2019-01-01 12:00:00'

没有索引（只有主键）

mysql > SELECT DISTINCT(k.base_id) FROM knowledge AS k LEFT JOIN knowledge_question AS q ON k.id=q.knowledge_id LEFT JOIN knowledge_answer AS a ON k.id=a.knowledge_id WHERE k.update_time>'2019-01-01 12:00:00' AND q.update_time>'2019-01-01 12:00:00' AND a.update_time>'2019-01-01 12:00:00';
+---------+
| base_id |
+---------+
|     159 |
...
|     413 |
|     414 |
+---------+
145 rows in set, 3 warnings (9 min 26.57 sec)

执行时间约10分钟，查看执行计划如下：

mysql > explain SELECT DISTINCT(k.base_id) FROM knowledge AS k LEFT JOIN knowledge_question AS q ON k.id=q.knowledge_id LEFT JOIN knowledge_answer AS a ON k.id=a.knowledge_id WHERE k.update_time>'2019-01-01 12:00:00' AND q.update_time>'2019-01-01 12:00:00' AND a.update_time>'2019-01-01 12:00:00';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                                        |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------------------------+
|  1 | SIMPLE      | k     | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 6238 |   100.00 | Using temporary                                              |
|  1 | SIMPLE      | q     | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 9540 |   100.00 | Using where; Distinct; Using join buffer (Block Nested Loop) |
|  1 | SIMPLE      | a     | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 8410 |   100.00 | Using where; Distinct; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------------------------+
3 rows in set, 4 warnings (0.00 sec)

全部都是全表扫描，根据MySQL联表查询的算法Nested-Loop Join，MySQL查询的结果集是3张表的笛卡尔积，所以效率特别低。

JOIN字段建立索引

explain SELECT DISTINCT(k.base_id) FROM knowledge AS k LEFT JOIN knowledge_question AS q ON k.id=q.knowledge_id LEFT JOIN knowledge_answer AS a ON k.id=a.knowledge_id WHERE k.update_time>'2019-01-01 12:00:00' AND q.update_time>'2019-01-01 12:00:00' AND a.update_time>'2019-01-01 12:00:00';
+----+-------------+-------+------------+------+---------------+---------+---------+-------------------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key     | key_len | ref               | rows | filtered | Extra                        |
+----+-------------+-------+------------+------+---------------+---------+---------+-------------------+------+----------+------------------------------+
|  1 | SIMPLE      | k     | NULL       | ALL  | PRIMARY       | NULL    | NULL    | NULL              | 6444 |    33.33 | Using where; Using temporary |
|  1 | SIMPLE      | a     | NULL       | ref  | idx_kid       | idx_kid | 4       | knowledge_base.k.id |    1 |    33.33 | Using where; Distinct        |
|  1 | SIMPLE      | q     | NULL       | ref  | idx_kid       | idx_kid | 4       | knowledge_base.k.id |    1 |    33.33 | Using where; Distinct        |
+----+-------------+-------+------------+------+---------------+---------+---------+-------------------+------+----------+------------------------------+

执行结果
+---------+
| base_id |
+---------+
|     159 |
...
|     413 |
|     414 |
+---------+
145 rows in set (0.02 sec)

耗时变成20毫秒

Where条件建立索引

给Where条件建立索引，并不一定会使用。比如：在表knowledge的字段update上建立索引idx_time：

MySQL [knowledge_base]> alter table knowledge add index idx_time(update_time);

MySQL [knowledge_base]> explain SELECT DISTINCT(k.base_id) FROM knowledge AS k LEFT JOIN knowledge_question AS q ON k.id=q.knowledge_id LEFT JOIN knowledge_answer AS a ON k.id=a.knowledge_id WHERE k.update_time>'2019-01-03 12:00:00' AND q.update_time>'2019-01-01 12:00:00' AND a.update_time>'2019-01-01 12:00:00';
+----+-------------+-------+------------+------+------------------+---------+---------+-------------------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys    | key     | key_len | ref               | rows | filtered | Extra                        |
+----+-------------+-------+------------+------+------------------+---------+---------+-------------------+------+----------+------------------------------+
|  1 | SIMPLE      | k     | NULL       | ALL  | PRIMARY,idx_time | NULL    | NULL    | NULL              | 6444 |    19.01 | Using where; Using temporary |
|  1 | SIMPLE      | a     | NULL       | ref  | idx_kid          | idx_kid | 4       | knowledge_base.k.id |    1 |    33.33 | Using where; Distinct        |
|  1 | SIMPLE      | q     | NULL       | ref  | idx_kid          | idx_kid | 4       | knowledge_base.k.id |    1 |    33.33 | Using where; Distinct        |
+----+-------------+-------+------------+------+------------------+---------+---------+-------------------+------+----------+------------------------------+

结果执行上来看，并没有使用索引idx_time。

如果where条件从k.update_time>'2019-01-03 12:00:00'修改为k.update_time='2019-01-03 12:00:00'（从>变成=）

MySQL [knowledge_base]> explain SELECT DISTINCT(k.base_id) FROM knowledge AS k LEFT JOIN knowledge_question AS q ON k.id=q.knowledge_id LEFT JOIN knowledge_answer AS a ON k.id=a.knowledge_id WHERE k.update_time='2019-01-03 12:00:00' AND q.update_time>'2019-01-01 12:00:00' AND a.update_time>'2019-01-01 12:00:00';
+----+-------------+-------+------------+------+------------------+----------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys    | key      | key_len | ref               | rows | filtered | Extra                 |
+----+-------------+-------+------------+------+------------------+----------+---------+-------------------+------+----------+-----------------------+
|  1 | SIMPLE      | k     | NULL       | ref  | PRIMARY,idx_time | idx_time | 4       | const             |    1 |   100.00 | Using temporary       |
|  1 | SIMPLE      | a     | NULL       | ref  | idx_kid          | idx_kid  | 4       | knowledge_base.k.id |    1 |    33.33 | Using where; Distinct |
|  1 | SIMPLE      | q     | NULL       | ref  | idx_kid          | idx_kid  | 4       | knowledge_base.k.id |    1 |    33.33 | Using where; Distinct |
+----+-------------+-------+------------+------+------------------+----------+---------+-------------------+------+----------+-----------------------+

则会使用索引idx_time

继续试验发现，如果在knowledge_question和knowledge_answer表上的字段update_time上建立索引，有时候会较大幅度的改变执行计划。所以说，检查SQL语句是否用到索引，一定要用explain查看执行计划，MySQL优化器做了太多的工作了。

其他知识点

在建立索引的时候，会遇到Table Metadata Lock的问题，可以先show processlist，找到占用表锁的连接，然后kill。

MySQL [(none)]> show processlist;
+---------+-----------+----------------------+--------------+---------+------+--------------+------------------------------------------------------------------------------------------------------+
| Id      | User      | Host                 | db           | Command | Time | State        | Info                                                                                                 |
+---------+-----------+----------------------+--------------+---------+------+--------------+------------------------------------------------------------------------------------------------------+
| 3468722 | Aics_user | 10.219.153.217:46574 | knowledge_base | Query   |   94 | Sending data | SELECT DISTINCT(k.base_id) FROM knowledge AS k LEFT JOIN knowledge_question AS q ON k.id=q |

MySQL [(none)]> kill 3468722