
```
select *
from (
select 'table' as table, toInt64(10) as rows, toInt64(101) as elements
union all
select 'another table' as table, toInt64(0) as rows, toInt64(0) as elements
)
where rows - elements <> 0

DB::Exception: Block structure mismatch in Pipe::unitePipes stream: different number of columns:
table String String(size = 0), rows Int64 Int64(size = 0), elements Int64 Int64(size = 0)
table String String(size = 0), rows Int64 Int64(size = 0), elements Int64 Int64(size = 0), dummy UInt8 UInt8(size = 0) (version 21.5.3.1 (official build))
```
{
    NodeRawConstPtrs required_nodes;
    required_nodes.reserve(required_names.size());
    std::unordered_map<std::string_view, const Node *> names_map;
    for (const auto * node : index)
        names_map[node->result_name] = node;
    for (const auto & name : required_names)
    {
        auto it = names_map.find(name);
        if (it == names_map.end())
            throw Exception(ErrorCodes::UNKNOWN_IDENTIFIER,
                            "Unknown column: {}, there are only columns {}", name, dumpDAG());
        required_nodes.push_back(it->second);
    }
    index.swap(required_nodes);
    removeUnusedActions();
}
void ActionsDAG::removeUnusedActions(bool allow_remove_inputs)