
```
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))
```
{
    struct Frame
    {
        const Node * node = nullptr;
        size_t next_child_to_visit = 0;
    };
    std::unordered_set<const Node *> split_nodes;
    std::unordered_set<const Node *> visited_nodes;
    std::stack<Frame> stack;
    
    for (const auto & node : nodes)
    {
        if (visited_nodes.count(&node))
            continue;
        visited_nodes.insert(&node);
        stack.push({.node = &node});
        while (!stack.empty())
        {
            auto & cur = stack.top();
            
            while (cur.next_child_to_visit < cur.node->children.size())
            {
                const auto * child = cur.node->children[cur.next_child_to_visit];