Function new_join_children

datafusion/physical-plan/src/projection.rs:847–882

If pushing down the projection over this join's children seems possible, this function constructs the new [`ProjectionExec`]s that will come on top of the original children of the join.

(
    projection_as_columns: &[(Column, String)],
    far_right_left_col_ind: i32,
    far_left_right_col_ind: i32,
    left_child: &Arc<dyn ExecutionPlan>,
    right_child: &Arc<dyn ExecutionPlan>,
)

/// this function constructs the new [`ProjectionExec`]s that will come on top
/// of the original children of the join.
pub fn new_join_children(
    projection_as_columns: &[(Column, String)],
    far_right_left_col_ind: i32,
    far_left_right_col_ind: i32,
    left_child: &Arc<dyn ExecutionPlan>,
    right_child: &Arc<dyn ExecutionPlan>,
) -> Result<(ProjectionExec, ProjectionExec)> {
    let new_left = ProjectionExec::try_new(
        projection_as_columns[0..=far_right_left_col_ind as _]
            .iter()
            .map(|(col, alias)| ProjectionExpr {
                expr: Arc::new(Column::new(col.name(), col.index())) as _,
                alias: alias.clone(),
            }),
        Arc::clone(left_child),
    )?;
    let left_size = left_child.schema().fields().len() as i32;
    let new_right = ProjectionExec::try_new(
        projection_as_columns[far_left_right_col_ind as _..]
            .iter()
            .map(|(col, alias)| {
                ProjectionExpr {
                    expr: Arc::new(Column::new(
                        col.name(),
                        // Align projected expressions coming from the right
                        // table with the new right child projection:
                        (col.index() as i32 - left_size) as _,
                    )) as _,
                    alias: alias.clone(),
                }
            }),
        Arc::clone(right_child),
    )?;

    Ok((new_left, new_right))
}

/// Checks three conditions for pushing a projection down through a join:
/// - Projection must narrow the join output schema.