State "Waiting for table flush" in processlist - mysql

Question

If I try to run queries (even as easy as select id from table limit 1 ) on some specific tables in a schema (only a few of them have this problem) I get stuck.
When looking at the processlist, the state is "Waiting for table flush".
Any suggestion about how do I unlock these tables so that I can query them?

Answer 1

Pileups of queries stuck in "Waiting for table flush" state are typically caused by something in the background excessively running "FLUSH TABLES" or ANALYZE TABLE statements. Most backup methods need to do this briefly when they begin, but if you are getting it all the time, the chances are that you have a process or a cron job somewhere doing this excessively. Find the source of this and disable it, and the problem should go away.

Answer 2

For MySQL
Identify the processes causing issues.
This command will help to detect processes waiting for disk I/O in D state:
watch "ps -eo pid,user,state,command | awk '\$3 == /D/ { print \$0 }'"
You can also seek processes with long runtime like this:
SHOW FULL PROCESSLIST\G
Fix the processes or related queries you previously detected.
If you need to kill the detected processes, you can find their ids with the previous full processlist and execute the command: KILL <pid>;
In case you want to fix the same issue For SQL :
Find wich processes are causing issues.
This query will list them:
SELECT TOP 20
qs.sql_handle,
qs.execution_count,
qs.total_worker_time AS Total_CPU,
total_CPU_inSeconds = --Converted from microseconds
qs.total_worker_time/1000000,
average_CPU_inSeconds = --Converted from microseconds
(qs.total_worker_time/1000000) / qs.execution_count,
qs.total_elapsed_time,
total_elapsed_time_inSeconds = --Converted from microseconds
qs.total_elapsed_time/1000000,
st.text,
qp.query_plan
FROM
sys.dm_exec_query_stats AS qs
CROSS APPLY
sys.dm_exec_sql_text(qs.sql_handle) AS st
CROSS APPLY
sys.dm_exec_query_plan (qs.plan_handle) AS qp
ORDER BY
qs.total_worker_time DESC
Fix the processes or related queries you previously detected.
If you need to kill the detected processes, you can find their ids with the previous full processlist and execute the command:
KILL <SPID>
GO
EXEC sp_who2
GO
You can find more alternatives and details at the following source questions/answers/comments:
https://serverfault.com/questions/316922/how-to-detect-the-process-and-mysql-query-that-makes-high-load-on-server How do I find out what is hammering my SQL Server?
You also can use step by step SQL related article: https://www.wearediagram.com/blog/terminating-sql-server-blocking-processes