大表查询必须使用索引

查询sql语句：
select Name ,Id, count(*)  from test where sesTime is not null group by Name order by count(*) desc limit 15;
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查看sql语句的执行计划：
desc select Name ,Id, count(*) from test where sesTime is not null group by Name order by count(*) desc limit 15;+----+-------------+----------------+------------+------+---------------+------+---------+------+---------+----------+----------------------------------------------+
| id | select_type | table          | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra                                        |
+----+-------------+----------------+------------+------+---------------+------+---------+------+---------+----------+----------------------------------------------+
|  1 | SIMPLE      | test | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 1305648 |    90.00 | Using where; Using temporary; Using filesort |
+----+-------------+----------------+------------+------+---------------+------+---------+------+---------+----------+----------------------------------------------+
1 row in set, 1 warning (0.00 sec)

Thu Oct 10 18:34:36 2019

显然，这是一个全表扫描语句，
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查看该表的数据量：
select count(*) from test;
+----------+
| count(*) |
+----------+
|  1373833 |
+----------+
1 row in set (0.48 sec)

Thu Oct 10 18:45:10 2019
显然，该表数据量有140万条记录左右。
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查看该表结构：
show create table mbi_order_temp\G;
*************************** 1. row ***************************

Thu Oct 10 18:48:10 2019
从这个表结构语句可知：分组字段fiveGradeName没有索引，因此整个语句都是全表扫描
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