HIVE-8435 introduces a new logical optimizer called IdentityProjectRemover, which may cause map join in spark branch to generate wrong plan.
Currently, the map join conversion in spark branch first goes through a method convertJoinMapJoin, which replaces a join op with a mapjoin op, removes RS associated with big table, and keep RSs for all small tables. Afterwards, in SparkReduceSinkMapJoinProc it replaces all parent RSs of the mapjoin op with HTS (note it doesn't check whether the RS belongs to small table or big table.)
The issue arises, when IdentityProjectRemover comes into play, which may result into a situation that a operator tree has two consecutive RSs. Imaging the following example:
In this case, all parents of the mapjoin op will be RS, even the branch for big table! In SparkReduceSinkMapJoinProc, they will be replaced with HTS, which is obviously incorrect.