使用相关子查询

标签：mysq   key   nbsp   pos   子查询   ble   执行计划   warnings   bsp   

truncate t_target;  
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);
        这个语句很长时间都出不来结果，只看一下执行计划吧。

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 | 997282 |   100.00 | Using where; Using temporary |
|  2 | DEPENDENT SUBQUERY | t2    | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 997282 |     1.00 | Using where                  |
+----+--------------------+-------+------------+------+---------------+------+---------+------+--------+----------+------------------------------+
2 rows in set, 3 warnings (0.00 sec)
        主查询和相关子查询都是全表扫描，一共要扫描100万*100万数据行，难怪出不来结果。
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使用相关子查询

标签：mysq   key   nbsp   pos   子查询   ble   执行计划   warnings   bsp   

原文地址：https://www.cnblogs.com/hyhy904/p/11311195.html