Built in function lock correlation

11.9　 Locking and unlocking function

MySQL Functions for locking and unlocking data are provided in , These functions include GET_LOCK(value,timeout)、RELEASE_LOCK(value)、IS_FREE_LOCK(value) and IS_USED_LOCK(value) function .

11.9.1　GET_LOCK(value,timeout) function

GET_LOCK(value,timeout) The function uses a string value Get lock with given name , continued timeout second . If the lock is acquired successfully , Then return to 1, If lock acquisition timeout , Then return to 0, If an error occurs , Then return to NULL. Use GET_LOCK(value,timeout) Lock obtained by function , When executed RELEASE_LOCK(value) Or disconnect the database （ Including normal disconnection and abnormal disconnection ）, The lock will be released . An example is as follows ：

---

mysql> SELECT GET_LOCK('mysql',1000);+------------------------+| GET_LOCK('mysql',1000) |+------------------------+|                      1 |+------------------------+1 row in set (0.00 sec)

---

Get a name of mysql, Duration is 1000s Lock of .

11.9.2　RELEASE_LOCK(value) function

RELEASE_LOCK(value) The function will take value Named lock release . If the cancellation is successful , Then return to 1, If the thread has not created a lock , Then return to 0, If the value The named lock does not exist , Then return to NULL.

---

mysql> SELECT RELEASE_LOCK('mysql');+-----------------------+| RELEASE_LOCK('mysql') |+-----------------------+|                     1 |+-----------------------+1 row in set (0.10 sec)

---

Be careful ： There are two cases when a lock does not exist .

· Never been GET_LOCK(value,timeout) Function obtained .

· The lock has been invoked RELEASE_LOCK(value) Function released .

11.9.3　IS_FREE_LOCK(value) function

IS_FREE_LOCK(value) The function judges with value Whether the named lock can be used . If it can be used , Then return to 1, If you can't use , That is to say, it is being used , Then return to 0, If an error occurs , Then return to NULL. An example is as follows ：

---

mysql> SELECT IS_FREE_LOCK('mysql');+-----------------------+| IS_FREE_LOCK('mysql') |+-----------------------+|                     1 |+-----------------------+1 row in set (0.00 sec)

---

11.9.4　IS_USED_LOCK(value) function

IS_USED_LOCK(value) The function judges with value Whether the named lock is being used , If it is being used , Then return the database connection using the lock ID, Otherwise return to NULL. An example is as follows ：

---

mysql> SELECT IS_USED_LOCK('mysql'), IS_USED_LOCK('test');+-----------------------+----------------------+| IS_USED_LOCK('mysql') | IS_USED_LOCK('test') |+-----------------------+----------------------+|                  NULL |                    8 |+-----------------------+----------------------+1 row in set (0.00 sec)