docs: def _initialise_database(self):
          """Creates the database schema.
  
          This will not create tables or indices that already exist and
          is safe to be called on an existing database.
  
          """
          self._logger.info('Creating database schema, if necessary')
          self._conn.execute(constants.CREATE_TABLE_TEXT_SQL)
          self._conn.execute(constants.CREATE_TABLE_TEXTNGRAM_SQL)
          self._conn.execute(constants.CREATE_TABLE_TEXTHASNGRAM_SQL)
          self._conn.execute(constants.CREATE_INDEX_TEXTHASNGRAM_SQL)
          self._conn.execute(constants.CREATE_INDEX_TEXT_SQL)