优化了一个mysql的语句，没有什么技术含量，说一下过程：
优化前语句如下，大约执行时间为1-2秒
select u.id ,u.s_user_name,u.s_user_img,(select count(a.n_copy_count) from t_fonts a
join t_practicerecords b on b.n_font_id=a.id where a.n_user_id=u.id and b.n_delete=0 group by a.n_user_id)as n_copy_count,case when (select count(id) from t_friendships
where n_user_id=11311 and n_friend_id=u.id)>0 then 'true' else 'false' end as is_collected from t_users u
where BINARY `s_user_name` = '做字小助手' order by LENGTH(s_user_name) asc ,id asc limit 0,10相关的执行计划如下：
mysql> EXPLAIN select u.id ,u.s_user_name,u.s_user_img,(select count(a.n_copy_count) from t_fonts a join t_practicerecords b on b.n_font_id=a.id where a.n_user_id=u.id and b.n_delete=0 group by a.n_user_id)as n_copy_count,case when (select count(id) from t_friendships where n_user_id=11311 and n_friend_id=u.id)>0 then 'true' else 'false' end as is_collected from t_users u where BINARY `s_user_name` = '做字小助手' order by LENGTH(s_user_name) asc ,id asc limit 0,10;
+----+--------------------+---------------+------------+--------+-------------------------------------------------------------+---------------------------------+---------+------------------+---------+----------+-----------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+---------------+------------+--------+-------------------------------------------------------------+---------------------------------+---------+------------------+---------+----------+-----------------------------+
| 1 | PRIMARY | u | NULL | ALL | NULL | NULL | NULL | NULL | 1360606 | 100.00 | Using where; Using filesort |
| 3 | DEPENDENT SUBQUERY | t_friendships | NULL | eq_ref | PRIMARY,PK__T_friend__3213E83F571DF1D5,user_index,n_user_id | PRIMARY | 8 | const,sykdb.u.id | 1 | 100.00 | NULL |
| 2 | DEPENDENT SUBQUERY | a | NULL | ref | PRIMARY,n_user_index | n_user_index | 4 | sykdb.u.id | 7 | 100.00 | Using index condition |
| 2 | DEPENDENT SUBQUERY | b | NULL | ref | idx_fid_score_delete_uploadtime | idx_fid_score_delete_uploadtime | 5 | sykdb.a.id,const | 5 | 100.00 | Using index |
+----+--------------------+---------------+------------+--------+-------------------------------------------------------------+---------------------------------+---------+------------------+---------+----------+-----------------------------+
4 rows in set, 4 warnings (0.00 sec)对t_users表的s_user_name字段创建索引，并修改表结构
mysql> create index idx_user_name on t_users (s_user_name);
Query OK, 0 rows affected (5.55 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table t_users modify s_user_name varchar(100) binary;
Query OK, 1474778 rows affected (45.23 sec)
Records: 1474778 Duplicates: 0 Warnings: 0修改语句把binary去掉，再次查询如下：
mysql> select u.id ,u.s_user_name,u.s_user_img,(select count(a.n_copy_count) from t_fonts a join t_practicerecords b on b.n_font_id=a.id where a.n_user_id=u.id and b.n_delete=0 group by a.n_user_id)as n_copy_count,case when (select count(id) from t_friendships where n_user_id=11311 and n_friend_id=u.id)>0 then 'true' else 'false' end as is_collected from t_users u where `s_user_name` = '做字小助手' order by LENGTH(s_user_name) asc ,id asc limit 0,10;
+----+-----------------+--------------------------------+--------------+--------------+
| id | s_user_name | s_user_img | n_copy_count | is_collected |
+----+-----------------+--------------------------------+--------------+--------------+
| 2 | 做字小助手 | /0/1/photo/20160729035328.jpeg | NULL | false |
+----+-----------------+--------------------------------+--------------+--------------+
1 row in set (0.00 sec)不过修改表结构需谨慎。
