SQL & PL/SQL

Hi,

I have a question regarding the function numtodsinterval.

Database version:

Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production

PL/SQL Release 12.2.0.1.0 - Production

"CORE    12.2.0.1.0    Production"

TNS for Linux: Version 12.2.0.1.0 - Production

NLSRTL Version 12.2.0.1.0 - Production

If I write the following statement I'm able to get an interval having 9 days precision:

select numtodsinterval(999999999,'day') intval
  from dual;
INTVAL                    
--------------------------
+999999999 00:00:00.000000

This is in line with Oracle documentation: NUMTODSINTERVAL which says:

By default, the precision of the return is 9.

A higher number of days (with 10 digits) will return error:

ORA-01873: the leading precision of the interval is too small

Now if I try to convert the corresponding hours (999999999 * 24 ) to an interval:

select numtodsinterval(999999999*24,'hour') intval
  from dual;
ORA-01873: the leading precision of the interval is too small

Isn't the resulting interval having the same precision? DAY(9) TO SECOND(6) ?

Can someone explain me the reason of this behavior?

Do I misunderstand the sentence "By default, the precision of the return is 9." in the documentation?

Regards,

Alberto