mysql sleep

今天看到了一个sql：

select count(*), sleep(5) from test

第一次看到这个sleep函数，所以专门研究了一波。这个函数的语法是：SLEEP(duration), 其中duration的单位是秒

Sleeps (pauses) for the number of seconds given by the duration argument, then returns 0. The duration may have a fractional part. If the argument is NULL or negative, SLEEP() produces a warning, or an error in strict SQL mode.

When sleep returns normally (without interruption), it returns 0:

简单的说他可以让sql在执行的时候sleep一段时间。

mysql> SELECT SLEEP(1000);
+-------------+
| SLEEP(1000) |
+-------------+
|           0 |
+-------------+

When SLEEP() is the only thing invoked by a query that is interrupted, it returns 1 and the query itself returns no error. This is true whether the query is killed or times out:

• This statement is interrupted using KILL QUERY from another session:

mysql> SELECT SLEEP(1000);
+-------------+
| SLEEP(1000) |
+-------------+
|           1 |
+-------------+

• This statement is interrupted by timing out:

mysql> SELECT /*+ MAX_EXECUTION_TIME(1) */ SLEEP(1000);
+-------------+
| SLEEP(1000) |
+-------------+
|           1 |
+-------------+

这里解释一下上面的语法，那个语法是Mysql Optimizer Hints

When SLEEP() is only part of a query that is interrupted, the query returns an error:

• This statement is interrupted using KILL QUERY from another session:

mysql> SELECT 1 FROM t1 WHERE SLEEP(1000);
ERROR 1317 (70100): Query execution was interrupted

• This statement is interrupted by timing out:

mysql> SELECT /*+ MAX_EXECUTION_TIME(1000) */ 1 FROM t1 WHERE SLEEP(1000);
ERROR 3024 (HY000): Query execution was interrupted, maximum statement
execution time exceeded

This function is unsafe for statement-based replication. A warning is logged if you use this function when binlog_format is set to STATEMENT.

参考资料

• mysql sleep function
• Mysql Optimizer Hints