
```
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))
```
{
    NodeRawConstPtrs required_nodes;
    required_nodes.reserve(required_names.size());
    NameSet added;
    for (const auto & node : index)
    {
        if (required_names.count(node->result_name) && added.count(node->result_name) == 0)
        {
            required_nodes.push_back(node);
            added.insert(node->result_name);
        }
    }
    if (added.size() < required_names.size())
    {
        for (const auto & name : required_names)
            if (added.count(name) == 0)
                throw Exception(ErrorCodes::UNKNOWN_IDENTIFIER,
                                "Unknown column: {}, there are only columns {}", name, dumpNames());
    }
    index.swap(required_nodes);
    removeUnusedActions();
}
void ActionsDAG::removeUnusedActions(const Names & required_names)