Enhancement

In TiFlash, the output schema of join is [left_columns, right_columns], for example

select a.id from a join b on a.id = b.id and a.id1 = b.id1 and a.id2 = b.id2 and a.id3 = b.id3

The output schema of join is [a.id, a.id1, a.id2, a.id3, b.id, b.id1, b.id2, b.id3]

However, only a.id is actually needed. And inside join, the cost of combine the left columns with the right columns is actually very high: for left columns, it need to call replicateRange, and for right columns, it need to be added row by row using AddFound, if we can only return the column used by the parent operators, it will avoid some meaningless cost and increase the performance of join.