code: def keys(self, index=None):
          
          if index is not None and index not in self._indexes:
              raise ValueError('Index {} does not exist'.format(index))
  
          db = self._indexes[index][0] if index else self._main_db
          with self._lmdb.begin(db=db) as txn:
              return [
                  key.decode()
                  for key in txn.cursor().iternext(keys=True, values=False)
              ]
docs: Returns a list of keys in the database