Comments on: Why Index could refuse to work ? http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/ Everything about MySQL Performance Sat, 21 Nov 2009 05:23:57 -0800 http://wordpress.org/?v=2.8.4 hourly 1 By: peter http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-586680 peter Tue, 16 Jun 2009 03:55:48 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-586680 Pete, This may happen if MySQL can't use this index for the query. FORCE INDEX only works if it is possible to use index but MySQL choses not to do that. Pete,

This may happen if MySQL can’t use this index for the query.
FORCE INDEX only works if it is possible to use index but MySQL choses not to do that.

]]> By: Peter Mouland http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-585952 Peter Mouland Mon, 15 Jun 2009 10:11:55 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-585952 Hi, any other reasons why FORCE INDEX is ignored? I have a table with 100 columns which about 25% are integers and most of those have indexes on them. A varchar column is being used as to left join to another table which also has an (btree) index on it, but for some reason it is not being used. EXPLAIN also doesn't tell me about any possible keys. The table has about 100,000 rows, so not huge but be steadily growing. any ideas?? thanks,pete Hi, any other reasons why FORCE INDEX is ignored? I have a table with 100 columns which about 25% are integers and most of those have indexes on them. A varchar column is being used as to left join to another table which also has an (btree) index on it, but for some reason it is not being used. EXPLAIN also doesn’t tell me about any possible keys. The table has about 100,000 rows, so not huge but be steadily growing. any ideas?? thanks,pete

]]> By: Suman http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-482355 Suman Thu, 19 Feb 2009 13:50:30 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-482355 Sorry this has been answered above, I missed it. Sorry this has been answered above, I missed it.

]]> By: Suman http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-482353 Suman Thu, 19 Feb 2009 13:47:37 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-482353 What will happen if the table is: CREATE TABLE `article` ( `article_id` int(5) NOT NULL, `dummy` varchar(255) NOT NULL DEFAULT 'dummy', PRIMARY KEY (`article_id`) ) And query is: SELECT * FROM article force INDEX (PRIMARY) WHERE article_id='10'; (single quotes) Will the index be used? What will happen if the table is:
CREATE TABLE `article` (
`article_id` int(5) NOT NULL,
`dummy` varchar(255) NOT NULL DEFAULT ‘dummy’,
PRIMARY KEY (`article_id`)
)
And query is:
SELECT * FROM article force INDEX (PRIMARY) WHERE article_id=’10′; (single quotes)
Will the index be used?

]]> By: Jonathan http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-391436 Jonathan Tue, 25 Nov 2008 06:49:45 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-391436 Thank you, this is a great post. It just caught me in a system! Thank you, this is a great post. It just caught me in a system!

]]> By: 记录/分享/…… » Blog Archive » A Few Common Performance Mistakes http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-249466 记录/分享/…… » Blog Archive » A Few Common Performance Mistakes Fri, 07 Mar 2008 01:37:51 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-249466 [...] http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/ [...] [...] http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/ [...]

]]> By: Big DBA Head! - Database Brain Power! » A Few Common Performance Mistakes http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-249316 Big DBA Head! - Database Brain Power! » A Few Common Performance Mistakes Thu, 06 Mar 2008 16:20:12 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-249316 [...] http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/ [...] [...] http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/ [...]

]]> By: peter http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-142336 peter Tue, 03 Jul 2007 14:08:57 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-142336 No this one is actually fine, because there is only one number for given string "10" is 10 but it is not the case other way around 10 can correspond to "10" "010" "10.0" and a lot of other strings. No this one is actually fine, because there is only one number for given string “10″ is 10 but it is not the case other way around 10 can correspond to “10″ “010″ “10.0″ and a lot of other strings.

]]> By: parvesh http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-142289 parvesh Tue, 03 Jul 2007 12:14:21 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-142289 Sorry for commenting on an old post. But recently, I saw another interesting thing. You are right about indexes not being used in this case, but it works the other way round. If the column is an integer and the query is something like SELECT * FROM article WHERE article_id="10"; My first intuition was that this will also fail, but MySQL is smart enough :) Sorry for commenting on an old post. But recently, I saw another interesting thing. You are right about indexes not being used in this case, but it works the other way round. If the column is an integer and the query is something like

SELECT * FROM article WHERE article_id=”10″;

My first intuition was that this will also fail, but MySQL is smart enough :)

]]> By: peter http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-2347 peter Fri, 08 Sep 2006 19:25:43 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-2347 I did not mention it in the article but a lot of such problems also come from comparing incompatible columns, ie varchar to integer which may require closer look to figure out. I would really like MySQL EXPLAIN to issue warning in such case or something similar so it is easier to track it down. Yes this mistake is usually done by people having limited experience, at least with MySQL. I did not mention it in the article but a lot of such problems also come from comparing incompatible columns, ie varchar to integer which may require closer look to figure out. I would really like MySQL EXPLAIN to issue warning in such case or something similar so it is easier to track it down.

Yes this mistake is usually done by people having limited experience, at least with MySQL.

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