HowtofindandkillmisbehavingMySQLqueries_MySQL

Sometimes the complexity of a relational database system can be overwhelming. Fortunately, that complexity is an advantage, as with MySQL's tools for managing queries. In this tutorial, I will show you how to find and kill any misbehaving MySQL queries .

To view the currently-running queries, log in to the MySQL console and run the 'show processlist' command:

mysql> show processlist;

+--------+--------+-----------------+---------+---------+-------+-------+------------------+-----------+---------------+-----------+| Id | User | Host| db| Command | Time| State | Info | Rows_sent | Rows_examined | Rows_read |+--------+--------+-----------------+---------+---------+-------+-------+------------------+-----------+---------------+-----------+|78233 | root | 127.0.0.1:37527 | mysql | Sleep | 16474 | | NULL | 6 | 6 | 6 ||84546 | root | 127.0.0.1:48593 | mysql | Sleep | 13237 | | NULL | 2 | 2 | 2 || 107083 | root | 127.0.0.1:56451 | mysql | Sleep | 15488 | | NULL | 1 | 121 | 121 || 131455 | root | 127.0.0.1:48550 | NULL| Query | 0 | NULL| show processlist | 0 | 0 | 0 |+--------+--------+-----------------+---------+---------+-------+-------+------------------+-----------+---------------+-----------+4 rows in set (0.03 sec)

The first column you should look at is 'Time', which is the number of seconds the process has been "doing the thing it's doing". A process whose command is 'Sleep' is waiting for a query to come in, so it's not consuming any resources. For any other process, however, a 'Time' of more than a few seconds indicates a problem.

In this case, the only query running is our 'show processlist' command. Let's see what it looks like if we have a poorly-written query running:

mysql> show processlist;

+--------+--------+-----------------+-----------+---------+-------+--------------+----------------------------------+-----------+---------------+-----------+| Id | User | Host| db| Command | Time| State| Info | Rows_sent | Rows_examined | Rows_read |+--------+--------+-----------------+-----------+---------+-------+--------------+----------------------------------+-----------+---------------+-----------+|78233 | root | 127.0.0.1:37527 | example | Sleep | 18046 || NULL | 6 | 6 | 6 ||84546 | root | 127.0.0.1:48593 | example | Sleep | 14809 || NULL | 2 | 2 | 2 || 107083 | root | 127.0.0.1:56451 | example | Sleep | 17060 || NULL | 1 | 121 | 121 || 132033 | root | 127.0.0.1:54642 | example | Query |27 | Sending data | select max(subtotal) from orders | 0 | 0 | 0 || 133933 | root | 127.0.0.1:48679 | NULL| Query | 0 | NULL | show processlist | 0 | 0 | 0 || 134122 | root | 127.0.0.1:49264 | example | Sleep | 0 || NULL | 0 | 0 | 0 |+--------+--------+-----------------+-----------+---------+-------+--------------+----------------------------------+-----------+---------------+-----------+6 rows in set (0.00 sec)

Ah! Now we see there is a query that's been running for almost 30 seconds. If we don't want to let it run its course, we can kill it by passing its 'Id' to the kill command:

mysql> kill 132033;

Query OK, 0 rows affected (0.00 sec)

mysql>

(Note that MySQL will always report 0 rows affected, because we're not altering any data.)

Judicious use of the kill command can clean up a backlog of queries. Remember, however, that it's not a permanent solution - if those queries came from your application, you need to rewrite them, or you'll continue to see the same issue reappear.

## See Also

MySQL's documentation on the different 'Command' values:

https://dev.mysql.com/doc/refman/5.7/en/thread-commands.html

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