> > It seems to me that the expressions "=" and "IN" are equivalent here due > to the fact that the aggregated subquery returns only one value, and the > result with the "IN" operation can be considered as the intersection of > elements on the left and right. In this query, we have some kind of set on > the left, among which there will be found or not only one element on the > right. >
Yes, they are equivalent at the final result, but there are some differences at the execution level. the '=' case will be transformed to a Subplan whose subPlanType is EXPR_SUBLINK, so if there is more than 1 rows is returned in the subplan, error will be raised. select * from tenk1 where ten = (select ten from tenk1 i where i.two = tenk1.two ); ERROR: more than one row returned by a subquery used as an expression However the IN case would not. select * from tenk1 where ten = (select ten from tenk1 i where i.two = tenk1.two ) is OK. I think the test case you added is not related to this feature. the difference is there even without the patch. so I kept the code you changed, but not for the test case. I took the liberty of adding this to your patch and added myself as >> reviewer, if you don't mind. >> > Sure, the patch after your modification looks better than the original. > I'm not sure how the test case around "because of got one row" is > relevant to the current changes. After we reach to some agreement > on the above discussion, I think v4 is good for committer to review! > > > Thank you!) I am ready to discuss it. > Actually I meant to discuss the "Unfortunately, I found a request..", looks we have reached an agreement there:) -- Best Regards Andy Fan
