3、如何针对具体的SQL做优化？
* 使用Explain分析SQL语句执行计划
mysql> explain select * from t_online_group_records where UNIX_TIMESTAMP(gre_updatetime) > 123456789;
+----+-------------+------------------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------------------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t_online_group_records | ALL | NULL | NULL | NULL | NULL | 47 | Using where |
+----+-------------+------------------------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
如上面例子所示，重点关注下type，rows和Extra：
type：使用类别，有无使用到索引。结果值从好到坏：... > range(使用到索引) > index > ALL(全表扫描)，一般查询应达到range级别
rows：SQL执行检查的记录数
Extra：SQL执行的附加信息，如"Using index"表示查询只用到索引列，不需要去读表等
* 使用Profiles分析SQL语句执行时间和消耗资源
mysql> set profiling=1; (启动profiles，默认是没开启的)
mysql> select count(1) from t_online_group_records where UNIX_TIMESTAMP(gre_updatetime) > 123456789; (执行要分析的SQL语句)
mysql> show profiles;
+----------+------------+----------------------------------------------------------------------------------------------+
| Query_ID | Duration | Query |
+----------+------------+----------------------------------------------------------------------------------------------+
| 1 | 0.00043250 | select count(1) from t_online_group_records where UNIX_TIMESTAMP(gre_updatetime) > 123456789 |
+----------+------------+----------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> show profile cpu,block io for query 1; (可看出SQL在各个环节的耗时和资源消耗)
+----------------------+----------+----------+------------+--------------+---------------+
| Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
...
| optimizing | 0.000016 | 0.000000 | 0.000000 | 0 | 0 |
| statistics | 0.000020 | 0.000000 | 0.000000 | 0 | 0 |
| preparing | 0.000017 | 0.000000 | 0.000000 | 0 | 0 |
| executing | 0.000011 | 0.000000 | 0.000000 | 0 | 0 |
| Sending data | 0.000076 | 0.000000 | 0.000000 | 0 | 0 |
...
* SQL优化的技巧 (只提一些业务常遇到的问题)
1、最关键：索引，避免全表扫描。
对接触的项目进行慢查询分析，发现TOP10的基本都是忘了加索引或者索引使用不当，如索引字段上加函数导致索引失效等(如where UNIX_TIMESTAMP(gre_updatetime)>123456789)
+----------+------------+---------------------------------------+
| Query_ID | Duration | Query |
+----------+------------+---------------------------------------+
| 1 | 0.00024700 | select * from mytable where id=100 |
| 2 | 0.27912900 | select * from mytable where id+1=101 |
+----------+------------+---------------------------------------+
另外很多同学在拉取全表数据时，喜欢用select xx from xx limit 5000,1000这种形式批量拉取，其实这个SQL每次都是全表扫描，建议添加1个自增id做索引，将SQL改为select xx from xx where id>5000 and id<6000;
+----------+------------+-----------------------------------------------------+
| Query_ID | Duration | Query |
+----------+------------+-----------------------------------------------------+
| 1 | 0.00415400 | select * from mytable where id>=90000 and id<=91000 |
| 2 | 0.10078100 | select * from mytable limit 90000,1000 |
+----------+------------+-----------------------------------------------------+
合理用好索引，应该可解决大部分SQL问题。当然索引也非越多越好，过多的索引会影响写操作性能
2、只se