Minor Description
FULL OUTER sort merge join (non-code-gen path) copies join keys and buffers input rows, even when rows from both sides do have matched keys (https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/SortMergeJoinExec.scala#L1637-L1641 ). This is unnecessary, as we can just output the row with smaller join keys, and only buffer when both sides have matched keys. This would save us from unnecessary copy and buffer, when both join sides have a lot of rows not matched with each other.
