
```
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 * node : index)
        if (node->result_name == column_name)
            return true;
    for (auto it = nodes.rbegin(); it != nodes.rend(); ++it)
    {
        auto & node = *it;
        if (node.result_name == column_name)
        {
            index.push_back(&node);
            return true;
        }
    }
    return false;
}
bool ActionsDAG::removeUnusedResult(const std::string & column_name)
{
    
    const Node * col;
    {
        auto it = index.begin();
        for (; it != index.end(); ++it)
            if ((*it)->result_name == column_name)
                break;
        if (it == index.end())
            throw Exception(ErrorCodes::LOGICAL_ERROR, "Not found result {} in ActionsDAG\n{}", column_name, dumpDAG());
        col = *it;
        index.erase(it);