Comments on: Why Index could refuse to work ? http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/ Percona's MySQL & InnoDB performance and scalability blog Sat, 11 Feb 2012 16:45:54 +0000 hourly 1 http://wordpress.org/?v=3.2.1 By: Mark Cotner http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-874905 Mark Cotner Fri, 13 Jan 2012 20:33:53 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-874905 This came up in a google search for a problem I was having where even if I forced I couldn't get it to use an index. I quietly read the ENTIRE article, assuming this couldn't possibly be it . . . surely we're smarter than that. Went back and checked anyway, sure enough . . . varchar for a universal_id field. I guess some of them must have characters in them(we don't assign them). Added quotes to the where clause and sped it up 300x. Thanks for posting this. It may seem obvious, but I surely didn't catch it. This came up in a google search for a problem I was having where even if I forced I couldn’t get it to use an index. I quietly read the ENTIRE article, assuming this couldn’t possibly be it . . . surely we’re smarter than that.

Went back and checked anyway, sure enough . . . varchar for a universal_id field. I guess some of them must have characters in them(we don’t assign them). Added quotes to the where clause and sped it up 300x.

Thanks for posting this. It may seem obvious, but I surely didn’t catch it.

]]> By: mylesmg http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/comment-page-1/#comment-693218 mylesmg Wed, 09 Dec 2009 20:13:51 +0000 http://www.mysqlperformanceblog.com/2006/09/08/why-index-could-refuse-to-work/#comment-693218 One reason not in this post, but in another post is that if you are joining tables, the join may not use an index if the tables are of different types (e.g. joining an InnoDB table to a Memory table). Simply changing the memory to innodb or vice versa will eliminate the issue. One reason not in this post, but in another post is that if you are joining tables, the join may not use an index if the tables are of different types (e.g. joining an InnoDB table to a Memory table). Simply changing the memory to innodb or vice versa will eliminate the issue.

]]> 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.

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