建立sex索引：
ALTER TABLE user ADD INDEX index_sex (sex) ;
SELECT * FROM user WHERE area='美国ATT用户' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.87s
SELECT * FROM user WHERE area='泰国' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.87s
SELECT * FROM user WHERE area='台湾省台湾大宽频' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.87s
SELECT * FROM user WHERE area='美国弗吉尼亚州' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.89s
SELECT * FROM user WHERE area='德国奔驰汽车' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.88s
SELECT * FROM user WHERE area='台湾省中华电信' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.87s
SELECT * FROM user WHERE area='韩国' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.86s
SELECT * FROM user WHERE area='拉美地区' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.88s
SELECT * FROM user WHERE area='美国纽约(Prudential)' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.87s
SELECT * FROM user WHERE area='印度尼西亚' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.87s
共花费时间：8.73s
同样能够提升性能（5.66s/8.73 约为0.6483倍）效率反而降低了？？求解？
建立这个sex索引还不如不建。

删除索引：
ALTER TABLE user DROP INDEX index_sex;
发现时间又变成了0.57s左右，

建立两个单独的索引：
ALTER TABLE user
ADD INDEX index_area (area) ,
ADD INDEX index_last_login (last_login) ;

SELECT * FROM user WHERE area='美国ATT用户' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.09s
SELECT * FROM user WHERE area='泰国' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.33s
SELECT * FROM user WHERE area='台湾省台湾大宽频' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.21s
SELECT * FROM user WHERE area='美国弗吉尼亚州' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.01s
SELECT * FROM user WHERE area='德国奔驰汽车' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.28s
SELECT * FROM user WHERE area='台湾省中华电信' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.02s
SELECT * FROM user WHERE area='韩国' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.02s
SELECT * FROM user WHERE area='拉美地区' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.02s
SELECT * FROM user WHERE area='美国纽约(Prudential)' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.03s
SELECT * FROM user WHERE area='印度尼西亚' AND sex=0 ORDER BY lastlogin DESC limit 30; 0.67s

发现建立两个单独的索引还不如只建立一个索引
删除索引：
发现时间又变成了0.57s左右，