Nobody to explain me that?
From: "mel list_php" <[EMAIL PROTECTED]> To: mysql@lists.mysql.com Subject: mysql index cardinality Date: Fri, 25 Feb 2005 16:47:12 +0000
Hi,
A strange thing with index, I thought the cardinality was automatically updated (like for a primary key for exemple).
When I use a primary key in a table, insert a row, the cardinality is increased of 1 as well.
I just tried to do that with an INDEX, and the cardinality is none unless I update it with analyze table for example.(http://dev.mysql.com/doc/mysql/en/show-index.html)
I also tried with KEY (which is supposed to be an alias of index) and after the first insertion it updated the cardinality but not later on.
1/any explanation?is there a kind of random update from time to time?
2/ is that cardinality important to know? I read that big cardinality will ensure that the index is used for joins for example. Does MySQL check the "real" cardinality before querying?Or do I have to run an analyze table from time to time?
3/ a primary key is just a peculiar index, so why is that value updated?
Thanks for any explanation....
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