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Update docs for newly added locking functions (#14448) #14462

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Aug 8, 2023
Merged

Update docs for newly added locking functions (#14448) #14462

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Update docs for newly added locking functions
dveeden Aug 7, 2023
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dveeden Aug 7, 2023
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fix format
Oreoxmt Aug 7, 2023
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Add note about compatibility
dveeden Aug 7, 2023
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Oreoxmt Aug 8, 2023
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10 changes: 4 additions & 6 deletions functions-and-operators/locking-functions.md
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Expand Up @@ -12,16 +12,14 @@ TiDB supports most of the user-level [locking functions](https://dev.mysql.com/d
| Name | Description |
|:---------------------------------------------------------------------------------------------------------------------|:----------------------------------------------------------------------|
| [`GET_LOCK(lockName, timeout)`](https://dev.mysql.com/doc/refman/5.7/en/locking-functions.html#function_get-lock) | Acquires an advisory lock. The `lockName` parameter must be NO longer than 64 characters. Waits maximum `timeout` seconds before timing out and returns a failure. |
| [`RELEASE_LOCK(lockName)`](https://dev.mysql.com/doc/refman/5.7/en/locking-functions.html#function_release-lock) | Releases a previously acquired lock. The `lockName` parameter must be NO longer than 64 characters. |
| [`IS_FREE_LOCK(lockName)`](https://dev.mysql.com/doc/refman/5.7/en/locking-functions.html#function_is-free-lock) | Checks if a lock is free. |
| [`IS_USED_LOCK(lockName)`](https://dev.mysql.com/doc/refman/5.7/en/locking-functions.html#function_is-used-lock) | Checks if a lock is in use. If true, it returns the corresponding connection ID. |
| [`RELEASE_ALL_LOCKS()`](https://dev.mysql.com/doc/refman/5.7/en/locking-functions.html#function_release-all-locks) | Releases all locks held by the current session. |
| [`RELEASE_LOCK(lockName)`](https://dev.mysql.com/doc/refman/5.7/en/locking-functions.html#function_release-lock) | Releases a previously acquired lock. The `lockName` parameter must be NO longer than 64 characters. |

## MySQL compatibility

* The minimum timeout permitted by TiDB is 1 second, and the maximum timeout is 1 hour (3600 seconds). This differs from MySQL, where both 0 second and unlimited timeouts (`timeout=-1`) are permitted. TiDB will automatically convert out-of-range values to the nearest permitted value and convert `timeout=-1` to 3600 seconds.
* TiDB does not automatically detect deadlocks caused by user-level locks. Deadlocked sessions will timeout after a maximum of 1 hour, but can also be manually resolved by using `KILL` on one of the affected sessions. You can also prevent deadlocks by always acquiring user-level locks in the same order.
* Locks take effect on all TiDB servers in the cluster. This differs from MySQL Cluster and Group Replication where locks are local to a single server.

## Unsupported functions

* `IS_FREE_LOCK()`
* `IS_USED_LOCK()`
* `IS_USED_LOCK()` returns `1` if it is called from another session and is unable to return the ID of the process that is holding the lock.
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