Optimizing SQL Performance – The Art of Elimination

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