1.generated columns：可以定义一个带有函数表达的列
例1：
CREATE TABLE triangle (
sidea DOUBLE,
sideb DOUBLE,
sidec DOUBLE AS (SQRT(sidea * sidea + sideb * sideb))
);
INSERT INTO triangle (sidea, sideb) VALUES(1,1),(3,4),(6,8);
mysql>show create table triangle\G
*************************** 1. row ***************************
Table: triangle
Create Table: CREATE TABLE `triangle` (
`sidea` double DEFAULT NULL,
`sideb` double DEFAULT NULL,
`sidec` double GENERATED ALWAYS AS (sqrt(((`sidea` * `sidea`) + (`sideb` * `sideb`)))) VIRTUAL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
2.generated columns参数
virtual：没有存储列的值，不占存储空间，在5.7.8之前，虚拟列不支持索引，到5.7.8，Innodb引擎开始支持secondary indexse，不指定参数，默认是虚拟列，如例1；
stored：列真实存在，并需要存储空间，支持索引；
例2：
mysql>create table t3(c1 int,c2 int generated always as (c1+1) virtual,c3 int generated always as (c1+1) stored);
mysql>insert into t3(c1) values(1);
mysql>select * from t3;
+------+------+------+
| c1 | c2 | c3 |
+------+------+------+
| 1 | 2 | 2 |
+------+------+------+
添加索引：
mysql>alter table t3 add index index_c2(c2);
mysql>alter table t3 add index index_c3(c3);
查看表的状态信息：
mysql>show table status like 't3'\G
*************************** 1. row ***************************
Name: t3
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 1
Avg_row_length: 16384
Data_length: 16384
Max_data_length: 0
Index_length: 0
Data_free: 0
Auto_increment: NULL
Create_time: 2016-12-22 22:45:38
Update_time: 2016-12-22 22:40:49
Check_time: NULL
Collation: utf8_general_ci
Checksum: NULL
Create_options:
Comment:
查看执行计划：
mysql>desc select * from t3 where c2=2\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t3
partitions: NULL
type: ref
possible_keys: index_c2
key: index_c2
key_len: 5
ref: const
rows: 1
filtered: 100.00
Extra: NULL
1 row in set, 1 warning (0.00 sec)
mysql>explain select * from t3 where c2=2\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t3
partitions: NULL
type: ref
possible_keys: index_c2
key: index_c2
key_len: 5
ref: const
rows: 1
filtered: 100.00
Extra: NULL
1 row in set, 1 warning (0.00 sec)
也可以在虚拟列上再创建虚拟列：
mysql>alter table t3 add c4 int generated always as (c2+1) virtual;
mysql>alter table t3 drop c4;
