MySQL自古以来就不提供函数索引这么复杂的功能。那怎么在MySQL里面实现这样的功能呢？ 我们先来看看函数索引的概念。函数索引，也可称为表达式索引，也就是基于字段以特定函数（表达式）建立索引来提升查询性能之需。函数索引的优势在于更加精确的获取所需要的数据。
MySQL 5.7提供了一个新的特性，虚拟列，可以很完美的解决这个问题。
在介绍虚拟列之前，我们来看看在MySQL里面普通索引的范例。
示例表结构：
CREATE TABLE t1 (id INT ,rank INT, log_time DATETIME, nickname VARCHAR(64)) ENGINE INNODB; ALTER TABLE t1 ADD PRIMARY KEY (id), ADD KEY idx_rank (rank),ADD KEY idx_log_time (log_time);
mysql> select count(*) from t1; +----------+ | count(*) | +----------+ | 5000 | +----------+ 1 row in set (0.00 sec)
mysql> SELECT * FROM t1 WHERE DATE(log_time) = '2015-04-09'\G
*************************** 1. row ***************************
id: 95
rank: 24
log_time: 2015-04-09 05:53:13
nickname: test
*************************** 2. row ***************************
id: 3423
rank: 42
log_time: 2015-04-09 02:55:38
nickname: test
2 rows in set (0.01 sec)
mysql> explain SELECT * FROM t1 WHERE DATE(log_time) = '2015-04-09'\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t1
partitions: NULL
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 5000
filtered: 100.00
Extra: Using where
1 row in set, 1 warning (0.00 sec)
mysql> SELECT * FROM t1 WHERE log_time >= '2015-04-09 00:00:00' AND log_time <='2015-04-10 00:00:00'\G
*************************** 1. row ***************************
id: 3423
rank: 42
log_time: 2015-04-09 02:55:38
nickname: test
*************************** 2. row ***************************
id: 95
rank: 24
log_time: 2015-04-09 05:53:13
nickname: test
2 rows in set (0.00 sec)
mysql> explain SELECT * FROM t1 WHERE log_time >= '2015-04-09 00:00:00' AND log_time <= '2015-04-10 00:00:00'\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t1
partitions: NULL
type: range
possible_keys: idx_log_time
key: idx_log_time
key_len: 6
ref: NULL
rows: 2
filtered: 100.00
Extra: Using index condition
1 row in set, 1 warning (0.00 sec)ALTER TABLE t1 ADD COLUMN log_date DATE AS (DATE(log_Time)) stored, ADD KEY idx_log_date (log_date);
mysql> SELECT * FROM t1 WHERE log_date = '2015-04-09'\G
*************************** 1. row ***************************
id: 95
rank: 24
log_time: 2015-04-09 05:53:13
nickname: test
log_date: 2015-04-09
*************************** 2. row ***************************
id: 3423
rank: 42
log_time: 2015-04-09 02:55:38
nickname: test
log_date: 2015-04-09
2 rows in set (0.00 sec)
mysql> explain SELECT * FROM t1 WHERE log_date = '2015-04-09'\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: t1
partitions: NULL
type: ref
possible_keys: idx_log_date
key: idx_log_date
key_len: 4
ref: const
rows: 2
filtered: 100.00
Extra: NULL
1 row in set, 1 warning (0.00 sec)
