今天开发提出需求，让统计数据，一询问才得知表中的数据量已达亿级以上。具体的sql如下：
SELECT id_province_code,gender,age,COUNT(1),SUM(zy_days),SUM(zf),SUM(ybnje)FROM medicare WHERE zy_enter_date BETWEEN '-01-01 00:00:00' AND '-12-31 12:59:59' GROUP BY id_province_code,age,gender;
然后查看该sql的执行计划
mysql> explain SELECT id_province_code,gender,age,COUNT(1),SUM(zy_days),SUM(zf),SUM(ybnje) FROM medicare WHERE zy_enter_date BETWEEN '-01-01 00:00:00' AND '-12-31 12:59:59' GROUP BYid_province_code,age,gender;+----+-------------+--------------+------------+-------+-------------------------------------------+-------------------+---------+------+---------+----------+--------------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+-------+-------------------------------------------+-------------------+---------+------+---------+----------+--------------------------------------------------------+
| 1 | SIMPLE | medicare | NULL | range | idx_zy_enter_date,idx_province_age_gender | idx_zy_enter_date | 4 | NULL | 4836248 | 100.00 | Using index condition; Using temporary; Using filesort |
执行计划中“Extra”中竟然出现了“Using temporary; Using filesort ”，看到这种情况我们就得进行优化了，虽然“type”列出现了“range”。出现这种情况是因为sql语句中使用了“group by”或者是“order by ”，然后进行了文件排序。
接着，我们就需要给group by后面的字段建立索引了，mysql索引原则是最左匹配前缀原则，我们给“id_province_code,age,gender”三字段添加一个复合索引(按照最左匹配原则)：
alter table medicare add index idx_ipc_age_gener(id_province_code,age,gender);
但一查看表结构
KEY `idx_province_age_gender` (`id_province_code`,`gender`,`age`)
早期已经建立好了，但是，怎么还会出现“Using temporary; Using filesort”，查看官档发现，group by 默认是要排序的，所以即使我们添加了索引，还是会引起文件排序。这样，我们的解决方案是：强制关闭排序：order by null
最后我们根据官方文档的建议，进行了sql的最终优化：
mysql> mysql> explain SELECT id_province_code,gender,age,COUNT(1),SUM(zy_days),SUMM(ybnje) FROM medicare WHERE zy_enter_date BETWEEN '-01-01 00:00:00' AND '-12-31 12:59:59' GROUP BY id_province_code,age,gender order by null;+----+-------------+--------------+------------+-------+-------------------------------------------+-------------------+---------+------+---------+----------+----------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+-------+-------------------------------------------+-------------------+---------+------+---------+----------+----------------------------------------+
| 1 | SIMPLE | medicare | NULL | range | idx_zy_enter_date,idx_province_age_gender | idx_zy_enter_date | 4 | NULL | 4836248 | 100.00 | Using index condition; Using temporary |
+----+-------------+--------------+------------+-------+-------------------------------------------+-------------------+---------+------+---------+----------+----------------------------------------+
查看执行计划，发现没有文件排序了，但是还是有“using tempory”,别着急，这是不重要的，只要执行sql的性能提升了就可以了
最终，运行最终优化后的sql发现比没有强制关闭排序的sql，整整快了将近4个小时(表中数据将近3亿，没关闭排序前的sql我运行了将近4个小时，还在跑，但优化后只跑了几十秒)看来优化的综合评估是很重要的。
