optimizing mysql_ log slow queries - web development stuff

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Optimizing MySQL: Log Slow Queries

Posted in Tutorials on Jun 22nd, 2009 | 8 responses

Tagged as mysql, performance

This is a second part of MySQL optimizing tutorial mini series. As the title indicates, were going focus on MySQL slow queries and their logging. Well use

the official MySQL manual for version 5.0 as a refernce. After read of this post you should be able to set up logging of MySQL slow queries for further useand be familiar with its basic output.

What Is It?

As mentioned earlier, in the first Optimizing MySQL part, the logging and evaluation of MySQL slow queries is one of the most important optimization

techniques. The slow query log consists of all SQL statements that took more than long_query_time seconds to execute. The time to acquire the initialtable locks is not counted as execution time. By exploring a slow query log we can get very interesting data to analyze. Results of such analysis may have

positive consequences on MySQL performance. But before we start check something we must have it available.

Turn it ON

Logging of slow queries is builtin capability of mysqld (MySQL deamon) which physically makes these entries. To enable mysql log slow queries, we havetwo choices how to do that:

Start mysqld with option --log-slow-queries[=file_name] from the command line

Start mysqld with option log-slow-queries=1 in the MySQL configuration file (usually /usr/local/etc/my.cnf)

If you do not specify the optional parameter file_name in the first case, MySQL creates a new file with default name host_name-slow.log and startswrite slow queries in there. The second case does not allow you to specify the filename and slow queries are written into the slow_log files and are very

friendly checkable as a classic MySQL table of default mysql database. Actually, you can find slow queries files in the /var/db/mysql/mysql/ directory

in both cases. Before well move forward, it would be usefull to mention the fields of slow queries log.

Slow Log Table Fields

start_time: datetime when a query starts



user_host: host on which a query was executedquery_time: how long a query took

lock_time: how long a table lock took

row_sent: number of rows sent back to caller

row_examined: number of rows scanneddb: name of database on which a query was executed

last_insert_id: not applicable, used if AUTO_INCREMENT set up

insert_id: not applicable, used if AUTO_INCREMENT set up

server_id: mysql server identificationsql_text: text of sql statement (which actually causes the slow query)

As you can see from the fields above, an information about slow query is good enough to use it for optimization. Maybe I forgot explain one important detail

mentioned in the second paragraph of this post: the selection criteria for slow query entry is long_query_time option set in the mysql config file. I have this

option set to 5 seconds (long_query_time=5 right before the log_slow_queries in the my.cnf file) as shown below.

Summary

The slow query log can be used to find queries that take a long time to execute and are therefore candidates for optimization. However, examining a long

slow query log can become a difficult task. To make this easier, you can process the slow query log using the mysqldumpslow command to summarize thequeries that appear in the log or explore the slow_log table entries by means of classic phpMyAdmin web interface. Theres also CSV file in the/var/db/mysql/mysql/ directory available for data export and further manipulation.

8 Responses to Optimizing MySQL: Log Slow Queries

1. David says:

June 25, 2009 at 9:05 am

Could we use waits here? I forgot what the command is in PHP to initiate a wait. You could also sort the slow queries by which took the longest,

giving you a priority list.

2. Teddy says:June 25, 2009 at 9:23 pm

I dont know what exactly do you think about, but PHP uses the sleep() function to delay execution.

3. Shahriat Hossain says:July 5, 2009 at 4:27 pm

Nice post but you can also optimize your query using profiling system which is now a days most popular technique.

4. Teddy says:

July 5, 2009 at 11:16 pm

Yes Shahriat, I agree with you but another optimizing techniques are getting ready, so stay tuned! Nevertheless, any suggestion for new parts of this

MySQL optimization mini series is appreciated as well!

5. web design sussex says:July 6, 2009 at 1:18 am



I am new to PHP having moved over from .NET. Im glad I read this article as I have many queries that seem to run slow in our CMS package which

have been identified using the slow log process. I just need to know how to optimize them now! Great post thanks!

6. sonny says:May 27, 2010 at 1:52 pm

How come insert the slow log queries info in slow log table as shown above?

7. Luke says:

August 31, 2010 at 6:30 pm

I have the same query in multiple places, I would like to find out what file the query is in in the log. Is this possible?

8. Teddy says:

August 31, 2010 at 6:40 pm

Yes, of course its possible. Use some file searching utility grep for an instance

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