Method removeUnusedColumns

src/Processors/QueryPlan/JoinStepLogical.cpp:397–549

}

JoinStepLogical::RemoveUnusedColumnsResult JoinStepLogical::removeUnusedColumns(const std::vector<size_t> & required_output_positions, bool remove_inputs)
{
    auto & actions_dag = *expression_actions.getActionsDAG();
    const size_t original_input_count = actions_dag.getInputs().size();
    ActionsDAG::NodeRawConstPtrs required_nodes;
    ActionsDAG::NodeRawConstPtrs new_actions_after_join = actions_after_join;

    /// For JoinStepLogical, the output header maps directly to DAG outputs (no pass-throughs).
    /// Build a set of required DAG output positions.
    std::set<size_t> required_positions_set(required_output_positions.begin(), required_output_positions.end());

    /// Track which original output positions survive (required + non-removable like dummy).
    std::vector<size_t> kept_output_positions;
    kept_output_positions.reserve(required_output_positions.size());

    bool removed_any_output = false;
    const auto & dag_outputs = actions_dag.getOutputs();
    for (size_t i = 0; i < dag_outputs.size(); ++i)
    {
        const auto * output_node = dag_outputs[i];
        if (required_positions_set.contains(i))
        {
            required_nodes.push_back(output_node);
            kept_output_positions.push_back(i);
        }
        else if (!isDummyColumnOfThisStep(output_node))
        {
            /// Do not remove join_dummy_result from the outputs, because it was added to ensure at least one output column
            removed_any_output = true;
            new_actions_after_join.erase(
                std::remove(new_actions_after_join.begin(), new_actions_after_join.end(), output_node), new_actions_after_join.end());
        }
        else
        {
            /// Dummy column kept even though not required.
            required_nodes.push_back(output_node);
            kept_output_positions.push_back(i);
        }
    }

    if (required_nodes.empty())
    {
        auto column_type = std::make_shared<DataTypeUInt8>();
        auto column = column_type->createColumnConst(0, 0);
        const auto * node = &actions_dag.addColumn(std::move(column), column_type, String(join_dummy_result_name));
        new_actions_after_join.push_back(node);
        required_nodes.push_back(node);
        actions_dag.getOutputs().push_back(node);
        kept_output_positions.push_back(RemoveUnusedColumnsResult::NEWLY_ADDED_COLUMN_POSITION);
    }

    ActionsDAG::NodeRawConstPtrs new_outputs = required_nodes;

    for (const auto & join_action : join_operator.expression)
        required_nodes.push_back(join_action.getNode());

    for (const auto & join_action : join_operator.residual_filter)
        required_nodes.push_back(join_action.getNode());