Detailed explanation of special phenomena examples of sleep function in MySQL

Preface

The sleep system function in MySQL has few practical application scenarios and is generally used for experimental testing. Yesterday, when testing, I accidentally discovered a special phenomenon of the sleep function. If the sleep function is used in a query statement, the sleep time is related to the records returned.

As shown in the following test:

mysql> create table test(id int);
Query OK, 0 rows affected (0.03 sec)

mysql> select *, sleep(6) from test;
Empty set (0.00 sec)

mysql> insert into test values(1);
Query OK, 1 row affected (0.00 sec)

mysql> select * ,sleep(6) from test;
+------+----------+
| id | sleep(6) |
+------+----------+
| 1 | 0 |
+------+----------+
1 row in set (6.00 sec)

mysql> insert into test value(2);
Query OK, 1 row affected (0.01 sec)

mysql> select * ,sleep(6) from test;
+------+----------+
| id | sleep(6) |
+------+----------+
| 1 | 0 |
| 2 | 0 |
+------+----------+
2 rows in set (12.00 sec) 

Test summary:

If, select *, sleep(n) from table, if the table record is empty, there will be no sleep. If the table record is one, then the sleep time is 1*n. If the table record is 2, then the sleep time is: 2*n ............ and so on.

In the official documentation, 12.24 Miscellaneous Functions does not mention this phenomenon, and I really don't know how to explain this situation. I speculated on several scenarios, but all of them were denied. Let me record this issue for now.

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