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