At the same time, output streams maintain the order on the L_ORDERKEY column to meet the input requirement of the Merge Join operator. The parallelism operator above the Index Seek operator is repartitioning its input streams using the value of O_ORDERKEY. Because its input isn't sorted on the O_ORDERKEY column values and this is the join column in the Merge Join operator, the Sort operator between the parallelism and Merge Join operators make sure that the input is sorted for the Merge Join operator on the join columns.