http://www.mysql.com/doc/en/SHOW_VARIABLES.html
states that
record_rnd_buffer_size When reading rows in sorted order after a sort,
the rows are read through this buffer to avoid a disk seeks. Can improve
ORDER BY by a lot if set to a high value. As this is a thread-specific
variable, one should not set this big globally, but just change this
when running some specific big queries.
But in 4.0.15 I get this error when trying to use it
/usr/local/mysql/bin/mysqld: ERROR: unknown variable
'record_rnd_buffer_size=2M'
Maybe the documentation should be updated to remove
record_rnd_buffer_size.
