High Performance My SQL, Third Edition
Consider the followingWHERE clause:
WHERE eye_color IN(‘brown‘,‘blue‘,‘hazel‘)
AND hair_color IN(‘black‘,‘red‘,‘blonde‘,‘brown‘)
AND sex IN(‘M‘,‘F‘)
The optimizer will convert this into 4*3*2 = 24 combinations, and the W
HERE clause will then have to check for each of them.
原文：http://www.cnblogs.com/yuanjiangw/p/6214352.html