def select(self, *args: str, _sys: bool = True) -> "Self":
        """Select only a specified set of signals."""
        new_schema = self.signals_schema.resolve(*args)
        if self._sys and _sys:
            new_schema = SignalSchema({"sys": Sys}) | new_schema
        columns = new_schema.db_signals()
        return self._evolve(
            query=self._query.select(*columns), signal_schema=new_schema
        )
Select only a specified set of signals.
src/datachain/lib/dc/datachain.py