测试验证datatime字段是否走索引

测试验证datatime字段是否走索引？

(product)root@localhost [lots]> show create table t_order\G

*************************** 1. row ***************************

Table: t_order

Create Table: CREATE TABLE `t_order` (

`ID` int(11) NOT NULL AUTO_INCREMENT COMMENT 'ID',

`CUSTOMER_ID` varchar(50) DEFAULT NULL COMMENT '客户号',

.....................................................................

`CREATE_DATE` datetime DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',

.....................................................................

PRIMARY KEY (`ID`),

.....................................................................

KEY `index_create_date` (`CREATE_DATE`) USING BTREE

) ENGINE=InnoDB AUTO_INCREMENT=2360435 DEFAULT CHARSET=gbk COMMENT='订单表'

1 row in set (0.00 sec)

查看下面有走索引：

(product)root@localhost [lots]> explain select customer_id from t_order where create_date='2015-10-19 21:03:40';

+----+-------------+---------+------+-------------------+-------------------+---------+-------+------+-------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+---------+------+-------------------+-------------------+---------+-------+------+-------+

| 1 | SIMPLE | t_order | ref | index_create_date | index_create_date | 6 | const | 1194 | NULL |

+----+-------------+---------+------+-------------------+-------------------+---------+-------+------+-------+

1 row in set (0.00 sec)

(product)root@localhost [lots]> explain select customer_id from t_order where create_date=date_format('2015-10-19 21:03:40',' %Y-%m-%d %H:%i:%S');

+----+-------------+---------+------+-------------------+-------------------+---------+-------+------+-------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+---------+------+-------------------+-------------------+---------+-------+------+-------+

| 1 | SIMPLE | t_order | ref | index_create_date | index_create_date | 6 | const | 1194 | NULL |

+----+-------------+---------+------+-------------------+-------------------+---------+-------+------+-------+

1 row in set (0.00 sec)

为什么不会存在字段类型的隐式转换呢？

当你需要同时包含日期和时间信息的值时则使用DATETIME类型。MySQL以'YYYY-MM-DD HH:MM:SS'格式检索和显示DATETIME值。支持的范围为'1000-01-01 00:00:00'到'9999-12-31 23:59:59'

。

说明mysql本身就支持这种显示格式，且只能说明是上面SQL发生类型转换的是在右边的值部分'2015-10-19 21:03:40'，只要左边字段不发生转换，就能用上索引。

测试oracle的行为：

alter session set nls_date_format='yyyy-mm-dd hh24:mi:ss';

SQL> explain plan for select * from zeng_test where datelock ='2009-3-11';

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------

Plan hash value: 822430127

--------------------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

--------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 4 | 52 | 4 (0)| 00:00:01 |

| 1 | TABLE ACCESS BY INDEX ROWID| ZENG_TEST | 4 | 52 | 4 (0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | DATELOCK_IX1 | 4 | | 1 (0)| 00:00:01 |

--------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

2 - access("DATELOCK"='2009-3-11')

14 rows selected

以上测试说明，mysql和oralce下只要左边字段不发生转换，就能用上索引。