SHOW VARIABLES Shows Variables MySQL Does Not Know About

Jan 8, 2008 / By Sheeri Cabral

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The listing of Dynamic System Variables on the MySQL Reference Manual’s page is supposed to show those variables you can change on-the-fly.

innodb_data_home_dir is listed there as a dynamic variable, but it is not one, and trying to change it on-the-fly doesn’t work:

mysql> set global innodb_data_home_dir="/data/mysql/data";
ERROR 1193 (HY000): Unknown system variable 'innodb_data_home_dir'

mysql> set session innodb_data_home_dir="/data/mysql/data";
ERROR 1193 (HY000): Unknown system variable 'innodb_data_home_dir'

The same goes when trying to use SET @@innodb_data_home_dir, and with log-slow-queries (changed to log_slow_queries as a parameter, of course). The irony is that innodb_data_home_dir shows up when SHOW VARIABLES is issued — so SHOW VARIABLES shows variables that MySQL is not aware of.

The link at the top is a link to the 4.1 version of the manual, just in case folks are thinking I’m using a newer manual version. Mind you, I have this problem with 5.0.45 as well as 4.1.20.

And of course the manual specifies, “The last column indicates for each variable whether GLOBAL or SESSION (or both) apply.” Which seems to be untrue.

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