Comments on: Obscure MySQL variable explained: max_seeks_for_key http://rackerhacker.com/2007/08/03/obscure-mysql-variable-explained-max_seeks_for_key/ Words of wisdom from a server administrator Wed, 20 Aug 2008 16:23:36 +0000 http://wordpress.org/?v=2.6 By: Georgi Kodinov http://rackerhacker.com/2007/08/03/obscure-mysql-variable-explained-max_seeks_for_key/#comment-20 Georgi Kodinov Fri, 31 Aug 2007 12:06:09 +0000 http://rackerhacker.com/2007/08/03/obscure-mysql-variable-explained-max_seeks_for_key/#comment-20 hmm : the formula is : max_seeks_for_key = <number of records in the table> / <cardinality=number of unique records>. Stupid HTML :( hmm : the formula is :
max_seeks_for_key = <number of records in the table> / <cardinality=number of unique records>.

Stupid HTML :(

]]> By: Georgi Kodinov http://rackerhacker.com/2007/08/03/obscure-mysql-variable-explained-max_seeks_for_key/#comment-19 Georgi Kodinov Fri, 31 Aug 2007 12:04:45 +0000 http://rackerhacker.com/2007/08/03/obscure-mysql-variable-explained-max_seeks_for_key/#comment-19 Indeed the documentation is not very clear about what max_seeks_for_key is. Actually it's not the cardinality. It is the number of rows you'll find for a unique value of the key. So this makes it : max_seeks_for_key = / . What limiting max_seeks_for_key means ? This means that it will still consider indexes that have very low cardinality to be considered otherwise. This is why it's not 1000 by default, but rather is set to the MAX_INT, so it can leave the optimizer to decide based on the statistics that it has. Indeed the documentation is not very clear about what max_seeks_for_key is.
Actually it’s not the cardinality. It is the number of rows you’ll find for a unique value of the key.
So this makes it :
max_seeks_for_key = / .
What limiting max_seeks_for_key means ? This means that it will still consider indexes that have very low cardinality to be considered otherwise.
This is why it’s not 1000 by default, but rather is set to the MAX_INT, so it can leave the optimizer to decide based on the statistics that it has.

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