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用 EXPLAIN 查看带视图的 SQL 执行计划

EXPLAIN 语句返回的结果会显示视图引用的表，而不是视图本身的名称。这是因为视图是一张虚拟表，本身并不存储任何数据。视图的定义会和查询语句的其余部分在 SQL 优化过程中进行合并。

示例

我们这里准备一个简单的示例，帮助你理解使用 EXPLAIN 解读 VIEW 的执行计划。

> drop table if exists t1;
> create table t1 (id int,ti tinyint unsigned,si smallint,bi bigint unsigned,fl float,dl double,de decimal,ch char(20),vch varchar(20),dd date,dt datetime);
> insert into t1 values(1,1,4,3,1113.32,111332,1113.32,'hello','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(2,2,5,2,2252.05,225205,2252.05,'bye','sub query','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(3,6,6,3,3663.21,366321,3663.21,'hi','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(4,7,1,5,4715.22,471522,4715.22,'good morning','my subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(5,1,2,6,51.26,5126,51.26,'byebye',' is subquery?','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(6,3,2,1,632.1,6321,632.11,'good night','maybe subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(7,4,4,3,7443.11,744311,7443.11,'yes','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(8,7,5,8,8758.00,875800,8758.11,'nice to meet','just subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(9,8,4,9,9849.312,9849312,9849.312,'see you','subquery','2022-04-28','2022-04-28 22:40:11');

> drop table if exists t2;
> create table t2 (id int,ti tinyint unsigned,si smallint,bi bigint unsigned,fl float,dl double,de decimal,ch char(20),vch varchar(20),dd date,dt datetime);
> insert into t2 values(1,1,4,3,1113.32,111332,1113.32,'hello','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(2,2,5,2,2252.05,225205,2252.05,'bye','sub query','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(3,6,6,3,3663.21,366321,3663.21,'hi','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(4,7,1,5,4715.22,471522,4715.22,'good morning','my subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(5,1,2,6,51.26,5126,51.26,'byebye',' is subquery?','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(6,3,2,1,632.1,6321,632.11,'good night','maybe subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(7,4,4,3,7443.11,744311,7443.11,'yes','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(8,7,5,8,8758.00,875800,8758.11,'nice to meet','just subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(9,8,4,9,9849.312,9849312,9849.312,'see you','subquery','2022-04-28','2022-04-28 22:40:11');

> create view v1 as select * from (select * from t1) sub where id > 4;
> select * from v1;
+------+------+------+------+----------+---------+------+--------------+----------------+------------+---------------------+
| id   | ti   | si   | bi   | fl       | dl      | de   | ch           | vch            | dd         | dt                  |
+------+------+------+------+----------+---------+------+--------------+----------------+------------+---------------------+
|    5 |    1 |    2 |    6 |    51.26 |    5126 |   51 | byebye       |  is subquery?  | 2022-04-28 | 2022-04-28 22:40:11 |
|    6 |    3 |    2 |    1 |    632.1 |    6321 |  632 | good night   | maybe subquery | 2022-04-28 | 2022-04-28 22:40:11 |
|    7 |    4 |    4 |    3 |  7443.11 |  744311 | 7443 | yes          | subquery       | 2022-04-28 | 2022-04-28 22:40:11 |
|    8 |    7 |    5 |    8 |     8758 |  875800 | 8758 | nice to meet | just subquery  | 2022-04-28 | 2022-04-28 22:40:11 |
|    9 |    8 |    4 |    9 | 9849.312 | 9849312 | 9849 | see you      | subquery       | 2022-04-28 | 2022-04-28 22:40:11 |
+------+------+------+------+----------+---------+------+--------------+----------------+------------+---------------------+
5 rows in set (0.01 sec)

如上述示例所示，新建了一个命名为 v1 的 VIEW，并查询 v1 的结果。那么我们看一下这个视图的执行过程：

>  explain select * from v1;
+----------------------------------+
| QUERY PLAN                       |
+----------------------------------+
| Project                          |
|   ->  Table Scan on db1.t1       |
|         Filter Cond: (t1.id > 4) |
+----------------------------------+
3 rows in set (0.01 sec)

可以看到 Project 为这次查询过程中的执行顺序的父节点，首先是从缩进最多的子节点开始计算，完成后“流入”它的上层父节点，最终“流入”Project 父节点。

先执行：

  • Filter Cond：过滤条件

再执行：

  • Table Scan：对某个全表进行扫描

下面的查询的执行方式与上述执行方式类似：

> explain select * from (select * from t1) sub where id > 4;
+----------------------------------+
| QUERY PLAN                       |
+----------------------------------+
| Project                          |
|   ->  Table Scan on db1.t1       |
|         Filter Cond: (t1.id > 4) |
+----------------------------------+
3 rows in set (0.00 sec)