Julian Hyde resolved CALCITE-2195.
    Resolution: Fixed

Fixed in 
thanks for the PR, [~zhong.j.yu]!

> AggregateJoinTransposeRule fails to aggregate over unique column
> ----------------------------------------------------------------
>                 Key: CALCITE-2195
>                 URL: https://issues.apache.org/jira/browse/CALCITE-2195
>             Project: Calcite
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 1.15.0
>            Reporter: Zhong Yu
>            Assignee: Julian Hyde
>            Priority: Major
>             Fix For: 1.16.0
> The following query, in which "A.sal" is unique,
> {code:java}
> select sum(A.sal)
> from (select distinct sal from sales.emp) as A
> join sales.emp as B on A.sal=B.sal
> {code}
>  causes AggregateJoinTransposeRule to fail with message
> {code:java}
> java.lang.AssertionError: type mismatch:
> aggCall type:
> inferred type:
> {code}
> 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.

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