Major Description
The following query, in which "A.sal" is unique,
select sum(A.sal) from (select distinct sal from sales.emp) as A join sales.emp as B on A.sal=B.sal
causes AggregateJoinTransposeRule to fail with message
java.lang.AssertionError: type mismatch: aggCall type: INTEGER inferred type: BIGINT
Apparently, this is a bug in the rule when `unique` is true on the A side, in which case the rule does not aggregate on the A side, `leftSubTotal==null`, causing `splitter.topSplit()` to only sum over `count()` coming from the B side.
A solution would be to introduce `splitter.singleton()` on the A side, so that it can be fed to topSplit() to be multiplied by the count.
In the case that the `unique` side does not contain the column of an agg call, it seems that we should do `other_singleton()` on this side, and feed it to topSplit(). However, realize that the `other()` expression is necessarily a `count()`, or a scalar function of `count()`, because it does not depend on any column values. In the same way, the proposed `other_singleton()` necessarily returns 1, or some constant value. `topSplit()` would not have any need of that constant value.Therefore in this case, we don't need a split on this side, just leave its subtotal as null.
I'm working on a pull-request based on these understandings. BTW, is there a reference to the algorithm used in the code? I can only find some old papers that don't exactly cover the logic of the code. Thanks.