On 4/22/2012 11:18 PM, Zhangzhigang wrote: > Why does not the mysql developer team to do this optimization? > > --- 12年4月20日，周五, Reindl Harald<h.rei...@thelounge.net> 写道： > >> ... >> >> because the mysql optimizer until now is really >> bad in many situations - order by rand() makes a >> temporary table wil ALL data as example even with limit >> >> select * from table order by rand() limit 10; >> reads and writes the whole table to disk >> have fun with large tables :-) >> >> >
When the Optimizer is told to sort a result set in the order determined by a random value created only at the time of the query, what better technique could they use than to materialize the table, sort the data, then return the results? If you can think of a better way of sorting random numbers, please tell us. MySQL has and still does accept solutions from the community. I do admit that at times in our past we have been very slow about processing those submissions but recently we have made great improvements in how we handle those. Also, we have made significant strides in improving our Optimizer in 5.5 and more improvements will be coming in 5.6. Please check out our newer versions to see if we have solved or improved any particular scalability problems you may be having. Regards, -- Shawn Green MySQL Principal Technical Support Engineer Oracle USA, Inc. - Hardware and Software, Engineered to Work Together. Office: Blountville, TN -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql
