Running reports from busy tables and avoiding locking those tables - mysql

Question

I have a number of reports involving joins on large datasets. These tables are being written to many times per second. My cronjobs run the queries at the least impactful times but still I am concerned about harming performance by locking tables with them.
Here is a simple example they requested as a one off today. It shows playtimes for a RIIA report:
SELECT
date_format(p.`played`, '%Y-%m') as `month`,
SUM(TIME_TO_SEC(s.`length`))/3600 as `playtime`
INTO OUTFILE "/tmp/120313_playtime.csv"
FIELDS TERMINATED BY ',' LINES TERMINATED BY '\n'
FROM
`plays` p,
`songs` s
GROUP BY `month`
How do I construct this to avoid causing issues for the radio app writing to the plays table while the query is running? Should I create temp tables and copy the live ones over?
// EDIT per request EXPLAIN output
+----+-------------+-------+------+---------------+------+---------+------+---------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+---------+---------------------------------+
| 1 | SIMPLE | s | ALL | NULL | NULL | NULL | NULL | 3909 | Using temporary; Using filesort |
| 1 | SIMPLE | p | ALL | NULL | NULL | NULL | NULL | 4040933 | Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+---------+---------------------------------+
CREATE TABLE `plays` (
`play_id` int(11) NOT NULL auto_increment,
`song_id` int(11) NOT NULL,
`played` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
PRIMARY KEY (`play_id`)
) ENGINE=MyISAM AUTO_INCREMENT=4040992 DEFAULT CHARSET=latin1 COMMENT='play counts for songs' AUTO_INCREMENT=4040992 ;
CREATE TABLE `songs` (
`id` int(11) NOT NULL auto_increment,
`title` varchar(255) NOT NULL,
`artist_id` int(11) NOT NULL,
`length` time NOT NULL,
`album_id` int(11) NOT NULL,
`active` tinyint(4) NOT NULL,
`tracknum` varchar(16) NOT NULL,
`bitrate` varchar(32) NOT NULL,
`date_created` datetime NOT NULL,
`date_modified` timestamp NOT NULL default '0000-00-00 00:00:00' on update CURRENT_TIMESTAMP,
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=4136 DEFAULT CHARSET=latin1 AUTO_INCREMENT=4136 ;

Answer 1

Just two immediate things come to mind... One, no "JOIN" between plays and songs which will result in a Cartesian product. Second, add a WHERE clause, and I would expect the "played" column is a date/time, so you could query for all records < NOW(), so if any are added while the query is running, they would be excluded. Since it appears you are doing monthly, you might even create a separate table that is nothing but the running totals per "time period" grouped by month and year, then you don't have to worry about a super long query. Then, you can just run for the current month in question... still less than NOW().