presto-main/src/main/java/io/prestosql/operator/scalar/StringFunctions.java/StringFunctions/charSubstr
class StringFunctions:    @Description("substring of given length starting at an index")
    @ScalarFunction("substr")
    @LiteralParameters("x")
    @SqlType("char(x)")
    public static Slice charSubstr(@SqlType("char(x)") Slice utf8, @SqlType(StandardTypes.BIGINT) long start, @SqlType(StandardTypes.BIGINT) long length)
    {
        return trimTrailingSpaces(substr(utf8, start, length));
    }