在 Oracle SQL 表中插入值的有效方法

Question

我有两张类似的桌子

 create table nodes_tbl as (
 select 'a' as nodeid, 'some string' as dummy_string, 0 as subnetid from dual union all
 select 'b', 'qwe', 0 from dual  union all
 select 'c', 'asd', 0  from dual union all
 select 'd', 'zxc', 0 from dual union all
 select 'e', 'rty', 0 from dual);

和

 create table subnets as (
 select 'a' as nodeid, 1 as subnetid from dual union all
 select 'b', 2 from dual  union all
 select 'c', 2 from dual union all
 select 'd', 3 from dual union all
 select 'e', 4 as nodeid from dual);

拥有数百万条记录的连接运行速度很快。

select  n.NODEID, n.DUMMY_STRING, s.subnetid
   from nodes_tbl n, subnets s where s.nodeid=n.nodeid 

写入速度也很快

create table test_tbl as  n.NODEID, s.subnetid 
 from nodes_tbl n, subnets s where s.nodeid=n.nodeid  --10M records in 2s.

但是，当我尝试更新表并向列添加值时，查询非常慢

      UPDATE nodes_tbl n
       SET subnetid = (SELECT subnetid
                             FROM subnets s
                            WHERE s.nodeid = n.nodeid)
    WHERE EXISTS (
    SELECT subnetid  FROM subnets s
                            WHERE s.nodeid = n.nodeid)  --8 minutes for 100K records

为什么插入比select 语句中的create table 慢得多？ 执行此插入的最有效方法是什么？

我知道创建视图选项，但想避免它。

Answer 1

改用MERGE：

merge into nodes_tbl n
  using (select s.subnetid, s.nodeid 
         from subnets s
        ) x
  on (x.nodeid = n.nodeid)
when matched then update set
  n.subnetid = x.subnetid;

有什么改善吗？

顺便问一下，您是否在两个表的NODEID 列上创建了索引？