And waits for them. That's pointless.
{
                bool was_renamed = false;
                bool was_removed = removed_columns.count(it->name);
                
                for (const auto & [rename_to, rename_from] : renamed_columns_to_from)
                {
                    if (rename_from == it->name)
                    {
                        was_renamed = true;
                        break;
                    }
                }
                
                
                if (renamed_columns_to_from.count(it->name) && !was_renamed && !was_removed)
                    throw Exception(
                        ErrorCodes::LOGICAL_ERROR,
                        "Incorrect mutation commands, trying to rename column {} to {}, but part {} already has column {}", renamed_columns_to_from[it->name], it->name, source_part->name, it->name);
                
                
                if (!renamed_columns_to_from.count(it->name) && (was_renamed || was_removed))
                    it = storage_columns.erase(it);
                else
                    ++it;
            }
        }
    }
    return storage_columns;
}
MergeTreeIndices MergeTreeDataMergerMutator::getIndicesForNewDataPart(