Function try_pushdown_through_join

datafusion/physical-plan/src/projection.rs:646–708

(
    projection: &ProjectionExec,
    join_left: &Arc<dyn ExecutionPlan>,
    join_right: &Arc<dyn ExecutionPlan>,
    join_on: JoinOnRef,
    schema: &SchemaRef,
    filter: Option<&JoinFilter>,
)

}

pub fn try_pushdown_through_join(
    projection: &ProjectionExec,
    join_left: &Arc<dyn ExecutionPlan>,
    join_right: &Arc<dyn ExecutionPlan>,
    join_on: JoinOnRef,
    schema: &SchemaRef,
    filter: Option<&JoinFilter>,
) -> Result<Option<JoinData>> {
    // Convert projected expressions to columns. We can not proceed if this is not possible.
    let Some(projection_as_columns) = physical_to_column_exprs(projection.expr()) else {
        return Ok(None);
    };

    let (far_right_left_col_ind, far_left_right_col_ind) =
        join_table_borders(join_left.schema().fields().len(), &projection_as_columns);

    if !join_allows_pushdown(
        &projection_as_columns,
        schema,
        far_right_left_col_ind,
        far_left_right_col_ind,
    ) {
        return Ok(None);
    }

    let new_filter = if let Some(filter) = filter {
        match update_join_filter(
            &projection_as_columns[0..=far_right_left_col_ind as _],
            &projection_as_columns[far_left_right_col_ind as _..],
            filter,
            join_left.schema().fields().len(),
        ) {
            Some(updated_filter) => Some(updated_filter),
            None => return Ok(None),
        }
    } else {
        None
    };

    let Some(new_on) = update_join_on(
        &projection_as_columns[0..=far_right_left_col_ind as _],
        &projection_as_columns[far_left_right_col_ind as _..],
        join_on,
        join_left.schema().fields().len(),
    ) else {
        return Ok(None);
    };

    let (new_left, new_right) = new_join_children(
        &projection_as_columns,
        far_right_left_col_ind,
        far_left_right_col_ind,
        join_left,
        join_right,
    )?;

    Ok(Some(JoinData {
        projected_left_child: new_left,