标签：create oss extra sql from target null select UNC
- 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。
建立created_time和item_name上的联合索引对比测试
标签：create oss extra sql from target null select UNC
原文地址：https://www.cnblogs.com/hyhy904/p/11311213.html
