…, while 8! = 40320. The worst-case complexity of MySQL’s join optimizer is N!. The goal of this choice was to…, that’s why the choice of higher search depth was left to the user who knows what they are doing. We…
Post: Using delayed JOIN to optimize count(*) and LIMIT queries
April 6, 2007 By 22 Comments
… for such result sets MySQL will perform the join even if you use LEFT JOIN so it is not needed which slows down… better performance you can “Help” MySQL and remove JOIN for count(*) and do JOIN after limiting result set for retrieval queries. Lets… select count(*) from fact left join dim on val=id where i select i,pad from fact left join dim on val=id where…
Post: Using CHAR keys for joins, how much is the overhead ?
June 18, 2007 By 18 Comments
…+t2.j+length(t2.c)+t1.j) from intjoin t1 left join intjoin t2 on t1.i=t2.j; +———————————-+ | sum(t1.i…+t2.j+length(t2.c)+t1.j) from intjoin t1 left join intjoin t2 on t1.i=t2.j \G *************************** 1. row… Tables may suffer significantly if key compression is not disabled Joining on Shorter CHAR keys is significantly faster than Long keys…
Post: Is there a performance difference between JOIN and WHERE?
April 14, 2010 By 25 Comments
… the answer. “Is there a performance difference between putting the JOIN conditions in the ON clause or the WHERE clause in… = B.ID; SELECT * FROM A JOIN B ON A.ID = B.ID; SELECT * FROM A JOIN B USING(ID); Personally, I prefer… from “comma joins” because a) the ANSI join syntax is more expressive and you’re going to use it anyway for LEFT JOIN, and…
Comment: Multi Column indexes vs Index Merge
October 20, 2009 By 30 Comments
… left join idling_events i on g.id = i.gps_position_id left join moving_events m on g.id = m.gps_position_id left join overspeed_events o on g.id = o.gps_position_id left join locations l on l.id = gl.location_id left join ios…
Comment: Why MySQL could be slow with large tables ?
April 12, 2007 By 139 Comments
…) INNER JOIN tblanswersets ASets USING (answersetid) INNER JOIN tblanswersetsanswers_x ASAX USING (answersetid) INNER JOIN tblanswers A USING (answerid) LEFT JOIN (tblevalanswerresults e1 INNER JOIN tblevaluations…) INNER JOIN tblanswersets ASets USING (answersetid) INNER JOIN tblanswersetsanswers_x ASAX USING (answersetid) INNER JOIN tblanswers A USING (answerid) LEFT JOIN (tblevalanswerresults e1 INNER JOIN tblevaluations…
Comment: ORDER BY ... LIMIT Performance Optimization
July 31, 2008 By 93 Comments
… last 2 left joins affect considerably to the performance. Any idea how I can optimize this query? Thanks. SELECT * FROM message LEFT JOIN accounts a ON message_account = a.id LEFT JOIN opportunities o ON message_project = o.id LEFT JOIN leads l1 ON…(message_from) LIKE CONCAT(\’%\’, UCASE(l1.email2), \’%\’) AND l1.email2 \’\') LEFT JOIN leads l2 ON (UCASE(message_to) LIKE CONCAT(\’%\’, UCASE(l2…
Comment: GROUP_CONCAT useful GROUP BY extension
May 11, 2009 By 43 Comments
… category FROM member_directory md LEFT JOIN member_categories mc ON md.id = mc.member_id LEFT JOIN categories ON mc.category_id = categories… category FROM member_directory md LEFT JOIN member_categories mc ON md.id = mc.member_id LEFT JOIN categories ON mc.category_id = categories…
Comment: ORDER BY ... LIMIT Performance Optimization
July 16, 2008 By 93 Comments
… forum_posts AS pa LEFT JOIN forum_posts_text AS pb ON pa.post_id = pb.post_id LEFT JOIN forum_users ON user… forum_posts AS pa LEFT JOIN forum_posts_text AS pb ON pa.post_id = pb.post_id LEFT JOIN forum_users ON user…
Comment: ORDER BY ... LIMIT Performance Optimization
July 9, 2007 By 93 Comments
… results: SELECT users.username,bla bla….. FROM whatever LEFT JOIN categories ON category = categories.id LEFT JOIN users ON torrents.owner = users.id WHERE…,1 SELECT users.username,bla bla FROM whatever LEFT JOIN categories ON category = categories.id LEFT JOIN users ON owner = users.id WHERE visible…
