Mysql – Why is this MySQL query taking so long

Rather than killing the process, it would be safer if you did it within MySQL:

$ mysqladmin processlist -u root -p
Enter password: 
+-----+------+-----------+-------------------+---------+------+-------+------------------+
| Id  | User | Host      | db                | Command | Time | State | Info             |
+-----+------+-----------+-------------------+---------+------+-------+------------------+
| 174 | root | localhost | example           | Sleep   | 297  |       |                  |
| 407 | root | localhost |                   | Query   | 0    |       | show processlist |
+-----+------+-----------+-------------------+---------+------+-------+------------------+

The query with id 174 is the one blocking deletion of the 'example' database, so before you kill any processes first let MySQL try to terminate the query:

$ mysqladmin kill 174

Run the processlist command above again to confirm that it was killed.

If this doesn't work, then you could perhaps look at killing the errant process, but before that you might try restarting the MySQL server.

You can also run commands like 'SHOW FULL PROCESSLIST' and 'KILL 174' in the MySQL shell, for example if you only have the MySQL client installed. The main point is to avoid killing the process using 'kill' in the shell unless absolutely necessary.

Generally speaking you can use either mysql or mysqladmin. You shouldn't need to be running commands like this that often though; once you start killing queries regularly something is definitely wrong and you'd be better off fixing that problem (killing the query process is just treating the symptom).

Look at the original query

select count(page_views.id) as views
from playpack_media
join page_views on page_views.itemId=playpack_media.media_Id
where playpack_media.playpack_id = 1
    and page_views.started_at BETWEEN '2014-06-23' and '2014-07-07'

You should do a couple of things.

First, create to compound indexes

ALTER TABLE page_views     ADD INDEX started_at_itemid_ndx    (started_at,itemid);
ALTER TABLE playpack_media ADD INDEX playpack_id_media_Id_ndx (playpack_id,media_Id)

Try to refactor the query

SELECT COUNT(1) as views FROM
(
    SELECT media_Id FROM playpack_media
    WHERE playpack_id = 1
) A INNER JOIN
(
    SELECT itemId FROM page_views WHERE
    started_at BETWEEN '2014-06-23' and '2014-07-07'
) B ON A.media_Id = B.itemId;

Perhaps breaking down the views by ItemID

SELECT IFNULL(A.media_id,'Total') Media,COUNT(1) as views FROM
(
    SELECT media_Id FROM playpack_media
    WHERE playpack_id = 1
) A INNER JOIN
(
    SELECT itemId FROM page_views WHERE
    started_at BETWEEN '2014-06-23' and '2014-07-07'
) B ON A.media_Id = B.itemId
GROUP BY A.media_id WITH ROLLUP;