Archive for July 8th, 2010

Optimizing SQL Performance – The Art of Elimination

Thursday, July 8th, 2010

The most efficient performance optimization of a SQL statement is to eliminate it. Cary Millsap’s recent Kaleidoscope presentation again highlighted that improving performance is function of code path. Removing code will improve performance.

You may think that it could be hard to eliminate SQL, however when you know every SQL statement that is executed in your code path obvious improvements may be possible. In the sequence SQL was implemented sometimes easy observations can lead to great gains. Let me provide some actual client examples that were discovered by using the MySQL General Log.

Example 1

5 Query   SELECT *  FROM `artist`
5 Query   SELECT *  FROM `artist`
5 Query   SELECT *  FROM `artist`
5 Query   SELECT *  FROM `artist`
5 Query   SELECT *  FROM `artist`
5 Query   SELECT *  FROM `artist` WHERE (ArtistID = 196 )
5 Query   SELECT *  FROM `artist` WHERE (ArtistID = 2188 )
5 Query   SELECT *  FROM `artist`
5 Query   SELECT *  FROM `artist`
5 Query   SELECT *  FROM `artist`

In this example, the following was executed for a single page load. Not only did I find a bug where full-table scans occurred rather then being qualified, there were many repeating and unnecessary occurrences.

Example 2

SELECT option_name, option_value FROM wp_options WHERE autoload = 'yes'
SELECT option_value FROM wp_options WHERE option_name = 'aiosp_title_format' LIMIT 1
SELECT option_value FROM wp_options WHERE option_name = 'ec3_show_only_even' LIMIT 1
SELECT option_value FROM wp_options WHERE option_name = 'ec3_num_months' LIMIT 1
SELECT option_value FROM wp_options WHERE option_name = 'ec3_day_length' LIMIT 1
SELECT option_value FROM wp_options WHERE option_name = 'ec3_hide_event_box' LIMIT 1
SELECT option_value FROM wp_options WHERE option_name = 'ec3_advanced' LIMIT 1
SELECT option_value FROM wp_options WHERE option_name = 'ec3_navigation' LIMIT 1
SELECT option_value FROM wp_options WHERE option_name = 'ec3_disable_popups' LIMIT 1
SELECT option_value FROM wp_options WHERE option_name = 'sidebars_widgets' LIMIT 1

This is a stock WordPress installation and highlights a classic Row at a Time (RAT) processing.

Example 3

SELECT * FROM activities_theme WHERE theme_parent_id=0
SELECT * FROM activities_theme WHERE theme_parent_id=1
SELECT * FROM activities_theme WHERE theme_parent_id=2
SELECT * FROM activities_theme WHERE theme_parent_id=11
SELECT * FROM activities_theme WHERE theme_parent_id=16

In this client example, again RAT processing, I provided a code improvement to run these multiple queries in a single statement, otherwise known as Chunk At a Time (CAT) processing. It’s not rocket science however the elimination of the network component of several SQL statements can greatly reduce page load time.

SELECT *
FROM   activities_theme
WHERE  theme_parent_id in  (0,1,2,11,16) 

Example 4

The following represents one of the best improvement. During capture, the following query was executed 6,000 times over a 5 minute period. While you make think this is acceptable, the value passed wae 0. The pages_id is an auto_increment column which by definition does not have a 0 value. In this instance, a simple boundary condition in the code would eliminate this query.

SELECT pages_id, pages_livestats_code, pages_title,
       pages_parent, pages_exhibid, pages_theme,
       pages_accession_num
FROM pages WHERE pages_id = 0

There are many tips to improving and optimizing SQL. This is the simplest and often overlooked starting point.

Related articles include: The RAT and the CAT and We need more CAT’s

What do MySQL Consultants do?

Thursday, July 8th, 2010

One role of a MySQL consultant is to review an existing production system. Sometimes you have sufficient time and access, and other times you don’t. If I am given a limited time here is a general list of things I look at.

  1. Review Server architecture, OS, Memory, Disks (including raid and partition type), Network etc
  2. Review server load and identify physical bottleneck
  3. Look at all running processes
  4. Look specifically at MySQL processes
  5. Review MySQL Error Log
  6. Determine MySQL version
  7. Look at MySQL configuration (e.g. /etc/my.cnf)
  8. Look at running MySQL Variables
  9. Look at running MySQL status (x n times)
  10. Look at running MySQL INNODB status (x n times) if used
  11. Get Database and Schema Sizes
  12. Get Database Schema
  13. Review Slow Query Log
  14. Capture query sample via SHOW FULL PROCESSLIST (locked and long running)
  15. Analyze Binary Log file
  16. Capture all running SQL

Here are some of the commands I would run.

2. Review server load and identify physical bottleneck

$ vmstat 5 720 > vmstat.`date +%y%m%d.%H%M%S`.txt

4. Look at MySQL processes

$ ps -eopid,fname,rss,vsz,user,command | grep -e "RSS" -e "mysql"
  PID COMMAND    RSS    VSZ USER     COMMAND
 5463 grep       764   5204 ronald   grep -e RSS -e mysql
13894 mysqld_s   596   3936 root     /bin/sh /usr/bin/mysqld_safe
13933 mysqld   4787812 5127208 mysql /usr/sbin/mysqld --basedir=/usr --datadir=/vol/mysql/mysqldata --user=mysql --pid-file=/var/run/mysqld/mysqld.pid --skip-external-locking --port=3306 --socket=/var/run/mysqld/mysqld.sock
13934 logger     608   3840 root     logger -p daemon.err -t mysqld_safe -i -t mysqld

$ ps -eopid,fname,rss,vsz,user,command | grep " mysqld " | grep -v grep | awk '{print $3,$4}'
4787820 5127208

5. Review MySQL Error Log

The error log can be found in various different places based on the operating system and configuration. It is important to find the right log, the SHOW GLOBAL VARIABLES LIKE ‘log_error’ will determine the location.

This is generally overlooked, however this can quickly identify some underlying problems with a MySQL environment.

7. Look at MySQL configuration

$ [ -f /etc/my.cnf ] &&  cat /etc/my.cnf
$ [ -f /etc/mysql/my.cnf ] &&  cat /etc/mysql/my.cnf
$ find / -name  "*my*cnf" 2>/dev/null

8. Look at running MySQL Variables

$ mysqladmin -uroot -p variables

9. Look at running MySQL status (x n times)

$ mysqladmin -uroot -p extended-status

It is important to run this several times at regular intervals, say 60 seconds, 60 minutes, or 24 hours.

I also have dedicated scripts that can perform this. Check out Log MySQL Stats.

11. Get Database and Schema Sizes

Check out my scripts on my MySQL DBA page

14. Capture Locked statements

Check out my script for Capturing MySQL sessions.

15. Analyze Binary Log file

Check out my post on using mk-query-digest.

16. Capture all SQL

Check out my post on DML Stats per table

Moving forward

Of course the commands I run exceeds this initial list, and gathering this information is only