
```
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))
```
visited_nodes.insert(node);
            visited_index_names.insert(node->result_name);
            stack.push(const_cast<Node *>(node));
        }
    }
    if (add_missing_keys)
    {
        for (const auto & column : required_columns)
        {
            if (visited_index_names.find(column) == visited_index_names.end())
            {
                if (const ColumnWithTypeAndName * column_with_type_name = projection_block_for_keys.findByName(column))
                {
                    const auto * node = &addInput(*column_with_type_name);
                    visited_nodes.insert(node);
                    index.push_back(node);
                    visited_index_names.insert(column);
                }
                else
                {
                    
                    return {};
                }
            }
        }
    }
    while (!stack.empty())
    {
        auto * node = stack.top();
        stack.pop();