
```
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))
```
ErrorCodes::THERE_IS_NO_COLUMN);
                }
                else
                {
                    src_node = dst_node = actions_dag->inputs[input.front()];
                    input.pop_front();
                }
                break;
            }
        }
        
        if (const auto * res_const = typeid_cast<const ColumnConst *>(res_elem.column.get()))
        {
            if (const auto * src_const = typeid_cast<const ColumnConst *>(dst_node->column.get()))
            {
                if (ignore_constant_values)
                    dst_node = &actions_dag->addColumn(res_elem);
                else if (res_const->getField() != src_const->getField())
                    throw Exception("Cannot convert column " + backQuote(res_elem.name) + " because "
                                    "it is constant but values of constants are different in source and result",
                                    ErrorCodes::ILLEGAL_COLUMN);
            }
            else
                throw Exception("Cannot convert column " + backQuote(res_elem.name) + " because "
                                "it is non constant in source stream but must be constant in result",
                                ErrorCodes::ILLEGAL_COLUMN);
        }
        
        if (!res_elem.type->equals(*dst_node->result_type))
        {