impl PhysicalExpr for CastExpr {
    fn evaluate(&self, df: &DataFrame, state: &ExecutionState) -> Result<Series> {
        let series = self.input.evaluate(df, state)?;
        series.cast_with_dtype(&self.data_type)
    }
    fn to_field(&self, input_schema: &Schema) -> Result<Field> {
        self.input.to_field(input_schema)
    }