[Enhancement] refactor to simplify the judge bottom-join

[jackwener]

https://github.com/ApsaraDB/galaxysql/blob/aa0b41ca6db291c0561089b5c25bd4171a969194/polardbx-calcite/src/main/java/org/apache/calcite/rel/rules/JoinCommuteRule.java#L190-L200

I think we don't need to judge it's bottom-join by using size() > 1.

Instead, we can just use left() instance of Join.

[jackwener]

How do you think? @Oldbread3

[chenzl25]

I don't think so. In VolcanoPlanner, the input of this logicalJoin could be a RelSubset instead of a specific Join node.

[Oldbread3]

agree with @chenzl25 , join could be hidden under any relNode(Project, Grouping, .etc), so that we need MetadataQuery to traverse whole sub-plan.

[jackwener]

I got it😀