Am 20.04.2012 04:29, schrieb 张志刚: > My point is that the limit can use late row lookup: lookup rows after > checking indexes to optimize the select speed. > > But the mysql optimizer do it with the early row lookup: lookup all rows > before checking indexes when the one fetch column is not in the indexes. > > Tell me why?
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 :-)
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