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 |
| -> Project |
| -> Project |
| -> Table Scan on db1.t1 |
| Filter Cond: (CAST(t1.id AS BIGINT) > 4) |
+--------------------------------------------------------------+
5 rows in set (0.00 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 |
| -> Project |
| -> Table Scan on db1.t1 |
| Filter Cond: (CAST(t1.id AS BIGINT) > 4) |
+--------------------------------------------------------+
4 rows in set (0.03 sec)