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)
Creates the database schema.

        This will not create tables or indices that already exist and
        is safe to be called on an existing database.