Question

I found some strange(for me) behavour in MySQL. I have a simple query:

SELECT CONVERT( `text`.`old_text` 
USING utf8 ) AS stext
FROM  `text` 
WHERE  `text`.`old_id` IN 
(    
 SELECT  `revision`.`rev_text_id`
 FROM  `revision` 
 WHERE  `revision`.`rev_id`
 IN 
  (    
  SELECT `page_latest`
  FROM  `page` 
  WHERE `page_id` = 108
  )
)

when i run it, phpmyadmin show execution time of 77.0446 seconds.
But then i replace

WHERE  `text`.`old_id` IN 

by

WHERE  `text`.`old_id` = 

it’s execution time falls to about 0.001 sec. Result of this query

 SELECT  `revision`.`rev_text_id`
 FROM  `revision` 
 WHERE  `revision`.`rev_id`
 IN 
  (    
  SELECT `page_latest`
  FROM  `page` 
  WHERE `page_id` = 108
  )    

is

+------------+
|rev_text_id |
+------------+
|6506        |
+------------+

Can somebody please explain this behavour?

Answer 1

There are two primary ways you can increase your query performance here

• Add Indexes (such as Kuya mentioned)
• Rid yourself of the subqueries where possible

For Indexes, add an index on the columns you are searching for your matches:
text.old_id, revision.rev_text_id & page.page_id

ALTER TABLE `text` ADD INDEX idx_text (old_id);
ALTER TABLE `revision` ADD INDEX idx_revision (rev_text_id);
ALTER TABLE `page` ADD INDEX idx_page (page_id);

Your next issue is that nested-sub-selects are hell on your query execution plan. Here is a good thread discussing JOIN vs Subquery. Here is an article on how to get execution plan info from mySQL.

First looks at an execution plan can be confusing, but it will be your best friend when you have to concern yourself with query optimization.

Here is an example of your same query with just joins ( you could use inner or left and get pretty much the same result). I don’t have your tables or data, so forgive synax issues (there is no way I can verify the code works verbatim in your environment, but it should give you a good starting point).

SELECT 
  CONVERT( `text`.`old_text` USING utf8 ) AS stext
  FROM  `text` 
  -- inner join only returns rows when it can find a 
  --    matching `revision`.`rev_text_id` row to `text`.`old_id`
  INNER JOIN `revision`  
  ON `text`.`old_id` = `revision`.`rev_text_id` 
  -- Inner Join only returns rows  when it can find a 
  --  matching `page_latest` row to `page_latest`
  INNER JOIN `page`
  ON `revision`.`rev_id` = `page`.`page_latest`
WHERE `page`.`page_id` = 108