performance - 什么更有效率 : several insert vs single insert with union

Question

我在 Postgresql 中有一个大表(~6M 行，41 列)，如下所示:

id | answer1 | answer2 | answer3 | ... | answer40
1  | xxx     | yyy     | null    | ... | null
2  | xxx     | null    | null    | ... | null
3  | xxx     | null    | zzz     | ... | aaa

注意每一行都有很多空列，我只想要那些有数据的

我想规范化它来得到这个:

id | answers
1  | xxx
1  | yyy
2  | xxx
3  | xxx
3  | zzz
...
3  | aaa

问题是，哪个更高效/更快，多个插入还是单个插入和多个联合？:

选项 1

create new_table as 
select id, answer1 from my_table where answer1 is not null
union 
select id, answer2 from my_table where answer2 is not null
union 
select id, answer3 from my_table where answer3 is not null
union ... 

选项 2

create new_table as select id, answer1 from my_table where answer1 is not null;
insert into new_table select id, answer2 from my_table where answer2 is not null;
insert into new_table select id, answer3 from my_table where answer3 is not null;
...

选项 3:有更好的方法吗？

Answer 1

选项 2 应该更快。

将所有语句包装在 begin-commit block 中以节省单独提交的时间。

为了更快的选择，确保被过滤的列(例如 where answer1 is not null)有索引