
```
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))
```
bool needed_by_split_node = false;
        bool visited = false;
        bool used_in_result = false;
        
        
        Node * to_second = nullptr;
        Node * to_first = nullptr;
    };
    std::stack<Frame> stack;
    std::unordered_map<const Node *, Data> data;
    for (const auto & node : index)
        data[node].used_in_result = true;
    
    for (const auto & node : nodes)
    {
        if (split_nodes.count(&node) == 0)
            continue;
        auto & cur_data = data[&node];
        if (cur_data.needed_by_split_node)
            continue;
        cur_data.needed_by_split_node = true;
        stack.push({.node = &node});
        while (!stack.empty())
        {
            auto & cur_node = stack.top().node;
            stack.pop();
            for (const auto * child : cur_node->children)
            {
                auto & child_data = data[child];
                if (!child_data.needed_by_split_node)