建立created_time和item_name上的联合索引对比测试

- 建立created_time和item_name字段的联合索引
create index idx_sort on t_source(created_time,item_name,item_id);  
analyze table t_source;
（1）使用相关子查询
truncate t_target;  （http://www.amjmh.com/v/BIBRGZ_558768/）
insert into t_target  
select distinct t1.* from t_source t1 where item_id in   
(select min(item_id) from t_source t2 where t1.created_time=t2.created_time and t1.item_name=t2.item_name);
        本次用时19秒，查询计划如下：

mysql> explain select distinct t1.* from t_source t1 where item_id in   
    -> (select min(item_id) from t_source t2 where t1.created_time=t2.created_time and t1.item_name=t2.item_name);  
+----+--------------------+-------+------------+------+---------------+----------+---------+----------------------------------------+--------+----------+------------------------------+
| id | select_type        | table | partitions | type | possible_keys | key      | key_len | ref                                    | rows   | filtered | Extra                        |
+----+--------------------+-------+------------+------+---------------+----------+---------+----------------------------------------+--------+----------+------------------------------+
|  1 | PRIMARY            | t1    | NULL       | ALL  | NULL          | NULL     | NULL    | NULL                                   | 997281 |   100.00 | Using where; Using temporary |
|  2 | DEPENDENT SUBQUERY | t2    | NULL       | ref  | idx_sort      | idx_sort | 89      | test.t1.created_time,test.t1.item_name |      2 |   100.00 | Using index                  |
+----+--------------------+-------+------------+------+---------------+----------+---------+----------------------------------------+--------+----------+------------------------------+
2 rows in set, 3 warnings (0.00 sec)
外层查询的t_source表是驱动表，需要扫描100万行。
对于驱动表每行的item_id，通过idx_sort索引查询出两行数据。