The bottom up approach in the tree diagram mirrors the flow of data during the execution of a query plan.

Rows are read in the leaf nodes of the query plan, WHERE clauses are applied to reduce the number of rows as soon as possible, with qualified rows being passed up through the remaining nodes of the query plan.

Intermediate plan nodes can sort the data or join it to rows from other tables.

Each successive node performs some refinement on the rows received from below. The final result rows emerge from the top of the plan as requested by the query.