Mysql – AWS RDS MySQL / Slow Query Log

amazon-rdsMySQL

I'm trying to analyze slow queries on my system and I read that I should enable the "Slow Query Log'. I curious how I can I view this log when I'm using an instance of MySQL hosted on Amazon Web Service's RDS.

Best Answer

Did you know there is slow log table? It is called mysql.slow_log:

mysql> show create table mysql.slow_log\G
*************************** 1. row ***************************
       Table: slow_log
Create Table: CREATE TABLE `slow_log` (
  `start_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `user_host` mediumtext NOT NULL,
  `query_time` time NOT NULL,
  `lock_time` time NOT NULL,
  `rows_sent` int(11) NOT NULL,
  `rows_examined` int(11) NOT NULL,
  `db` varchar(512) NOT NULL,
  `last_insert_id` int(11) NOT NULL,
  `insert_id` int(11) NOT NULL,
  `server_id` int(10) unsigned NOT NULL,
  `sql_text` mediumtext NOT NULL
) ENGINE=CSV DEFAULT CHARSET=utf8 COMMENT='Slow log'
1 row in set (0.01 sec)

mysql>

There are two variables you must activate to start recording slow queries in mysql.slow_log:

Here is the situation

mysql> show variables like 'log_output';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| log_output    | TABLE |
+---------------+-------+
1 row in set (0.01 sec)

mysql> show variables like 'slow_query_log';
+----------------+-------+
| Variable_name  | Value |
+----------------+-------+
| slow_query_log | OFF   |
+----------------+-------+
1 row in set (0.01 sec)

mysql>

log_output is already defaulted to TABLE. You will need to enable slow_query_log. Here is what you must do:

If your RDS Instance has no DB Parameter Group...

  1. Spin up an AmazonRDS MySQL Instance that uses a DB Parameter Group.
  2. Edit the DB Parameter Group by setting the slow_query_log to 1
  3. Restart the DB Instance

If your RDS Instance has DB Parameter Group...

  1. Edit the DB Parameter Group by setting the slow_query_log to 1
  2. Restart the DB Instance

Once the Instance has been restarted, login to mysql and run this query

mysql> SELECT SLEEP(15);

Since the default for long_query_time is 10 seconds, the query SELECT SLEEP(15); should show up in mysql.slow_log. Read the RDS Documentation to see if there are any limits on the table.

CAVEAT

Here is a fair warning. Notice mysql.slow_log is a CSV file. The table has no key. While you can query the table, it will do full table scans. It would be great if you can do the following:

ALTER TABLE mysql.slow_log ENGINE=MyISAM;
ALTER TABLE mysql.slow_log ADD INDEX (start_time);

Then, you can query very fast for date and time ranges. I tried to run this and I get this:

mysql> ALTER TABLE mysql.slow_log ENGINE=MyISAM;
ERROR 1044 (42000): Access denied for user 'lwdba'@'%' to database 'mysql'
mysql>

Check the RDS Documentation to see if Amazon can do this for you.

Give it a Try !!!

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