HIVE-8202 supports auto-conversion of SMB Join. However, if the tables are partitioned, there could be a slow down as each mapper would need to get a very small chunk of a partition which has a single key. Thus, in some scenarios it's beneficial to convert SMB join to map join.
The task is to research and support the conversion from SMB join to map join for Spark execution engine. See the equivalent of MapReduce in SortMergeJoinResolver.