Detecting locked tables (locked by LOCK TABLE)
MysqlLockingMysql Problem Overview
Is there a way to detect locked tables in MySQL? I mean tables locked by the LOCK TABLE table WRITE/READ command.
(Note that readers interested in detecting named locks acquired with GET_LOCK should read https://stackoverflow.com/q/11034504/1709587 instead.)
Mysql Solutions
Solution 1 - Mysql
Use SHOW OPEN TABLES:
http://dev.mysql.com/doc/refman/5.1/en/show-open-tables.html
You can do something like this
SHOW OPEN TABLES WHERE `Table` LIKE '%[TABLE_NAME]%' AND `Database` LIKE '[DBNAME]' AND In_use > 0;
to check any locked tables in a database.
Solution 2 - Mysql
You can use SHOW OPEN TABLES to show each table's lock status. More details on the command's doc page are here.
Solution 3 - Mysql
SHOW OPEN TABLES to show each table status and its lock.
For named locks see https://stackoverflow.com/questions/11034504/show-all-current-locks-from-get-lock
Solution 4 - Mysql
The simplest way is :
SHOW OPEN TABLES WHERE In_use > 0
You get the locked tables only of the current database.
Solution 5 - Mysql
You can create your own lock with GET_LOCK(lockName,timeOut)
If you do a GET_LOCK(lockName, 0) with a 0 time out before you lock the tables and then follow that with a RELEASE_LOCK(lockName) then all other threads performing a GET_LOCK() will get a value of 0 which will tell them that the lock is being held by another thread.
However this won't work if you don't have all threads calling GET_LOCK() before locking tables. The documentation for locking tables is here
Hope that helps!
Solution 6 - Mysql
You could also get all relevant details from performance_schema:
SELECT
OBJECT_SCHEMA
,OBJECT_NAME
,GROUP_CONCAT(DISTINCT EXTERNAL_LOCK)
FROM performance_schema.table_handles
WHERE EXTERNAL_LOCK IS NOT NULL
GROUP BY
OBJECT_SCHEMA
,OBJECT_NAME
This works similar as
show open tables WHERE In_use > 0
Solution 7 - Mysql
This article describes how to get information about locked MySQL resources. mysqladmin debug might also be of some use.
Solution 8 - Mysql
The following answer was written by Eric Leschinki in 2014/15 at https://stackoverflow.com/a/26743484/1709587 (now deleted):
> Mini walkthrough on how to detect locked tables:
> ------------------------------------------------
>
> This may prevent the database from enforcing atomicity in the affected tables and rows. The locks were designed to make sure things stay consistent and this procedure will prevent that process from taking place as designed.
>
> Create your table, insert some rows
>
> create table penguins(spam int, ham int);
> insert into penguins(spam, ham) values (3, 4);
>
> show open tables:
>
> show open tables like "penguins"
>
> prints:
>
> your_database penguins 0 0
>
> Penguins isn't locked, lets lock it:
>
> LOCK TABLES penguins READ;
>
> Check if it's locked:
>
> show open tables like "penguins"
>
> Prints:
>
> your_database, penguins 1, 0
>
> Aha! It is locked! Lets unlock it:
>
> unlock tables
>
> Now it is unlocked:
>
> show open tables like "penguins"
>
> Prints:
>
> your_database penguins 0 0
>
>
> show all current locks
> ----------------------
>
> show open tables where in_use <> 0
>
>
It would be much more helpful if the MySQL developers put this information in a regular table (so I can do a select my_items from my_table where my_clauses), rather than this stripped down 'show table' syntax from system variables.