从分组中选择第一行和分组数据

Question

我有两个单独的查询，我试图有效加入。

Query 1:
Select Id From Accounts Where Status='OPEN' and Product = 'Product A'


Query 2: 
Select AccountId From Transactions Group By AccountId Having Count(*) > 20;

表事务包含数百万行。

我想要实现的是将第一个帐户退回到处于未结状态且产品A超过20个交易的帐户。

到目前为止我得到了这个，但由于事务表的全表扫描，它并不是很有效：

Select A.Id From Accounts A 
Left Outer Join (Select AccountId From Transactions Group By AccountId Having Count(*) > 20 ) T on A.Id=T.AccountId
Where A.Status='OPEN' and A.Product = 'Product A'
And rownum = 1

如何优化此查询？

Answer 1

这会更好，过滤特定帐户和分组??

Select AccountId From Transactions where AccountId in (Select id From Accounts Where Status='OPEN' and Product = 'Product A') Group By AccountId Having Count(*) > 20;