Function flatten_join_inputs

datafusion/optimizer/src/eliminate_cross_join.rs:236–266

Recursively accumulate possible_join_keys and inputs from inner joins (including cross joins). Assumes can_flatten_join_inputs has returned true and thus the plan can be flattened. Adds all leaf inputs to `all_inputs` and join_keys to possible_join_keys

(
    plan: LogicalPlan,
    possible_join_keys: &mut JoinKeySet,
    all_inputs: &mut Vec<LogicalPlan>,
    all_filters: &mut Vec<Expr>,
)

/// flattened. Adds all leaf inputs to `all_inputs` and join_keys to
/// possible_join_keys
fn flatten_join_inputs(
    plan: LogicalPlan,
    possible_join_keys: &mut JoinKeySet,
    all_inputs: &mut Vec<LogicalPlan>,
    all_filters: &mut Vec<Expr>,
) -> Result<()> {
    match plan {
        LogicalPlan::Join(join) if join.join_type == JoinType::Inner => {
            if let Some(filter) = join.filter {
                all_filters.push(filter);
            }
            possible_join_keys.insert_all_owned(join.on);
            flatten_join_inputs(
                Arc::unwrap_or_clone(join.left),
                possible_join_keys,
                all_inputs,
                all_filters,
            )?;
            flatten_join_inputs(
                Arc::unwrap_or_clone(join.right),
                possible_join_keys,
                all_inputs,
                all_filters,
            )?;
        }
        _ => {
            all_inputs.push(plan);
        }
    };
    Ok(())
}

/// Returns true if the plan is a Join or Cross join could be flattened with
/// `flatten_join_inputs`