Select type&partitions (2)—mysql执行计划（四十八)

2023-02-02 14:40:01

前面说了explain的table是表名，显示在前面的代表驱动表，正常select会出现不同的id，但如果子查询本来是两个select，但被优化成连接查询，就会导致是相同的id，union查询会出现临时表，id为null，这个临时表作用于去重，union all不需要去重，所以也就不需要建立临时表。

Select type

我们都知道sql里会包含若干个select，每个select代表一个小的查询语句，每个select的from都可以关联若干张表，每张表对应执行计划输出一条数据，对于同一个select下面，id是相同的。

Mysql又为每个select定义了type，取名为select type，这样就知道每个select扮演什么样的角色。

Simple

查询语句不含包子查询或者union的都算simple类型，比方说下面的单表查询就是simple

mysql> EXPLAIN SELECT * FROM s1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | s1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 9688 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

当然连接查询也算simple，因为没子查询和union

mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------------+
|  1 | SIMPLE      | s1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 9688 |   100.00 | NULL                                  |
|  1 | SIMPLE      | s2    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 9954 |   100.00 | Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------------+
2 rows in set, 1 warning (0.01 sec)

Primary

对于union 和union all 都分为好几个select，其中左边的select就是primary类型

mysql> EXPLAIN SELECT * FROM s1 UNION SELECT * FROM s2;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | s1         | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 9688 |   100.00 | NULL            |
|  2 | UNION        | s2         | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 9954 |   100.00 | NULL            |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)

从上面可以看到s1查询的select_type就是primary类型。

Union

对于union和union all都是有好几个select组成，除了左边的是primary外，其他的都是union，从上面的例子就可以看到。

Union Result

当使用union去重时候，会创建临时表，这个临时表的select type就是union result。

SUBQUERY

如果子查询不能满足semi-join的查询条件，该子查询是不相关子查询，并且mysql优化器会选择物化方式执行sql，这时候子查询的select_type就是subquey

mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key1 FROM s2) OR key3 = 'a';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key      | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | s1    | NULL       | ALL   | idx_key3      | NULL     | NULL    | NULL | 9688 |   100.00 | Using where |
|  2 | SUBQUERY    | s2    | NULL       | index | idx_key1      | idx_key1 | 303     | NULL | 9954 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

可以看到外层的就是primary，子查询就是subquery，需要注意的是，子查询会被物化，所以只需要执行一次。

DEPENDENT SUBQUERY

如果包含子查询不能转成semi-join的形式，并且该子查询是相关子查询，这时候select type 就是dependent subquery

mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key1 FROM s2 WHERE s1.key2 = s2.key2) OR key3 = 'a';
+----+--------------------+-------+------------+------+-------------------+----------+---------+-------------------+------+----------+-------------+
| id | select_type        | table | partitions | type | possible_keys     | key      | key_len | ref               | rows | filtered | Extra       |
+----+--------------------+-------+------------+------+-------------------+----------+---------+-------------------+------+----------+-------------+
|  1 | PRIMARY            | s1    | NULL       | ALL  | idx_key3          | NULL     | NULL    | NULL              | 9688 |   100.00 | Using where |
|  2 | DEPENDENT SUBQUERY | s2    | NULL       | ref  | idx_key2,idx_key1 | idx_key2 | 5       | xiaohaizi.s1.key2 |    1 |    10.00 | Using where |
+----+--------------------+-------+------------+------+-------------------+----------+---------+-------------------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)

需要注意，因为s2是相关子查询，没有物化，所以需要执行多次。

DEPENDENT UNION

在包含union的或者union all的大查询中，各个小查询都依赖外层查询的话，除了左边那个小查询，其他小查询都属于dependent union

mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key1 FROM s2 WHERE key1 = 'a' UNION SELECT key1 FROM s1 WHERE key1 = 'b');
+----+--------------------+------------+------------+------+---------------+----------+---------+-------+------+----------+--------------------------+
| id | select_type        | table      | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra                    |
+----+--------------------+------------+------------+------+---------------+----------+---------+-------+------+----------+--------------------------+
|  1 | PRIMARY            | s1         | NULL       | ALL  | NULL          | NULL     | NULL    | NULL  | 9688 |   100.00 | Using where              |
|  2 | DEPENDENT SUBQUERY | s2         | NULL       | ref  | idx_key1      | idx_key1 | 303     | const |   12 |   100.00 | Using where; Using index |
|  3 | DEPENDENT UNION    | s1         | NULL       | ref  | idx_key1      | idx_key1 | 303     | const |    8 |   100.00 | Using where; Using index |
| NULL | UNION RESULT       | <union2,3> | NULL       | ALL  | NULL          | NULL     | NULL    | NULL  | NULL |     NULL | Using temporary          |
+----+--------------------+------------+------------+------+---------------+----------+---------+-------+------+----------+--------------------------+
4 rows in set, 1 warning (0.03 sec)

从上面可以看到，个外层查询是primary，子查询里面左边的是dependent subquery ，其余的子查询里都是dependent union

DERIVED

前面我们说的派生查询，大家还记得吗，当子查询在外层的from后面，即为派生查询，

mysql> EXPLAIN SELECT * FROM (SELECT key1, count(*) as c FROM s1 GROUP BY key1) AS derived_s1 where c > 1;
+----+-------------+------------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys | key      | key_len | ref  | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | <derived2> | NULL       | ALL   | NULL          | NULL     | NULL    | NULL | 9688 |    33.33 | Using where |
|  2 | DERIVED     | s1         | NULL       | index | idx_key1      | idx_key1 | 303     | NULL | 9688 |   100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

对于上面物化采用派生查询的方式,s1查询就是物化之后，派生查询，所以select_type是derived，上面的是以物化临时表查询的，所以table是derived2。(注意这里mysql优化器选的是物化查询，如果转成连接查询，就不会有临时表了)

materialized

当吧子查询物化之后，再把子查询与外层连接查询

mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key1 FROM s2);
+----+--------------+-------------+------------+--------+---------------+------------+---------+-------------------+------+----------+-------------+
| id | select_type  | table       | partitions | type   | possible_keys | key        | key_len | ref               | rows | filtered | Extra       |
+----+--------------+-------------+------------+--------+---------------+------------+---------+-------------------+------+----------+-------------+
|  1 | SIMPLE       | s1          | NULL       | ALL    | idx_key1      | NULL       | NULL    | NULL              | 9688 |   100.00 | Using where |
|  1 | SIMPLE       | <subquery2> | NULL       | eq_ref | <auto_key>    | <auto_key> | 303     | xiaohaizi.s1.key1 |    1 |   100.00 | NULL        |
|  2 | MATERIALIZED | s2          | NULL       | index  | idx_key1      | idx_key1   | 303     | NULL              | 9954 |   100.00 | Using index |
+----+--------------+-------------+------------+--------+---------------+------------+---------+-------------------+------+----------+-------------+
3 rows in set, 1 warning (0.01 sec)

这里可以看到，吧s2表物化了，物化之后，吧他们连接查询，所以是simple。

uncacheable subquery和uncacheable union不常用。