
```
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))
```
for (const auto * output : index)
        {
            if (node_to_column.count(output) == 0)
            {
                std::stack<Frame> stack;
                stack.push({.node = output});
                while (!stack.empty())
                {
                    auto & frame = stack.top();
                    const auto * node = frame.node;
                    while (frame.next_child < node->children.size())
                    {
                        const auto * child = node->children[frame.next_child];
                        if (node_to_column.count(child) == 0)
                        {
                            stack.push({.node = child});
                            break;
                        }
                        ++frame.next_child;
                    }
                    if (frame.next_child < node->children.size())
                        continue;
                    stack.pop();
                    ColumnsWithTypeAndName arguments(node->children.size());
                    for (size_t i = 0; i < arguments.size(); ++i)
                    {
                        arguments[i] = node_to_column[node->children[i]];
                        if (!arguments[i].column)
                            throw Exception(ErrorCodes::NOT_FOUND_COLUMN_IN_BLOCK,
                                            "Not found column {} in block", node->children[i]->result_name);