presto-main/src/main/java/io/prestosql/operator/scalar/MathFunctions.java/MathFunctions/truncate
class MathFunctions:    @Description("round to integer by dropping digits after decimal point")
    @ScalarFunction
    @SqlType(StandardTypes.DOUBLE)
    public static double truncate(@SqlType(StandardTypes.DOUBLE) double num)
    {
        return Math.signum(num) * Math.floor(Math.abs(num));
    }