一、案例描述

今日在進行MYSQL日常檢查時，發現有一條SQL語句，在欄位型別為datetime並建立索引的情況下，進行between (sysdate()+interval(-1) hour) and sysdate()時並不會使用索引，但是用now()代替sysdate就可以使用索引，這是為什麼呢？

二、問題

為什麼datetime欄位(已有且只有一個關鍵字的索引）與sysdate()進行比較運算時不使用索引，而用now()則會？

三、思路

不使用索引的情況一般是型別不對，那麼sysdate()返回的型別不是datetime？ 判斷sysdate()返回的型別

四、收集資料與分析

1. 確認sysdate(),now()的返回型別

create table qqt.t1 as select now();
create table qqt.t2 as select sysdate();
desc qqt.t1;
desc qqt.t2;

發現now()和sysdate()返回的都是datetime型別

2. sysdate()和now()有什麼區別呢？

SYSDATE()

Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS.uuuuuu format, depending on
 
 whether the function is used in a string or numeric context.

SYSDATE() returns the time at which it executes. This differs from the behavior for NOW(), which returns a constant time that indicates the time at which the statement began to execute. (Within a stored function or trigger, NOW() returns the
 
 time at which the function or triggering statement began to execute.)


mysql> SELECT NOW(), SLEEP(2), NOW();
+---------------------+----------+---------------------+
| NOW()               | SLEEP(2) | NOW()               |
+---------------------+----------+---------------------+
| 2006-04-12 13:47:36 |        0 | 2006-04-12 13:47:36 |
+---------------------+----------+---------------------+

mysql> SELECT SYSDATE(), SLEEP(2), SYSDATE();
+---------------------+----------+---------------------+
| SYSDATE()           | SLEEP(2) | SYSDATE()           |
+---------------------+----------+---------------------+
| 2006-04-12 13:47:44 |        0 | 2006-04-12 13:47:46 |
+---------------------+----------+---------------------+
In addition, the SET TIMESTAMP statement affects the value returned by NOW() but not by SYSDATE(). This means that timestamp settings in the binary log have no effect on invocations of SYSDATE().

Because SYSDATE() can return different values even within the same statement, and is not affected by SET TIMESTAMP, it is nondeterministic and therefore unsafe for replication if statement-based binary logging is used. If that is a problem, you can use row-based logging.

Alternatively, you can use the --sysdate-is-now option to cause SYSDATE() to be an alias for NOW(). This works if the option is used on both the master and the slave.

The nondeterministic nature of SYSDATE() also means that indexes cannot be used for evaluating expressions that refer to it.

根據官方說明已經很好理解了，因為sysdate()是該函式執行時動態獲取時間，NOW()是語句開始時就獲取時間，這時優化器進行SQL解釋時，已經能確認NOW()返回值，但sysdate()不能，所以不能評估成本，於是優化器只能對sysdate()選擇全表掃描，就now()就可以用索引了。

五、結論

• 對於索引關鍵字是datetime型別的欄位，使用sysdate進行比較運算時，由於優化器進行SQL評估時是無法獲知sysdate()值，而令優化器無法評估成本，所以只能全表掃描。
• 對於now()函式，由於優化器進行SQL執行計劃評估時就已經能確定返回的值，所以可以評估出索引還是全表掃描更合適。