
```
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))
```
index.clear();
    index.reserve(projection.size());
    for (const auto & item : projection)
    {
        auto it = names_map.find(item.first);
        if (it == names_map.end())
            throw Exception(ErrorCodes::UNKNOWN_IDENTIFIER,
                            "Unknown column: {}, there are only columns {}", item.first, dumpNames());
        index.push_back(it->second);
    }
    for (size_t i = 0; i < projection.size(); ++i)
    {
        const auto & item = projection[i];
        auto & child = index[i];
        if (!item.second.empty() && item.first != item.second)
        {
            Node node;
            node.type = ActionType::ALIAS;
            node.result_type = child->result_type;
            node.result_name = std::move(item.second);
            node.column = child->column;
            node.children.emplace_back(child);
            child = &addNode(std::move(node));
        }
    }
    removeUnusedActions();
    projectInput();
    projected_output = true;
}
bool ActionsDAG::tryRestoreColumn(const std::string & column_name)