Select Message_Text
  From (Select Rownum Rn, Count(1) Over() Total_Rn, a.* From X$dbgalertext a) Aa
 Where Rn + 100 >= Total_Rn

Count(1) Over() 

SQL> Select Message_Text
  2    From (Select Rownum Rn, Count(1) Over() Total_Rn, a.* From X$dbgalertext a) Aa
  3   Where Rn + 100 >= Total_Rn
  4  /
  From (Select Rownum Rn, Count(1) Over() Total_Rn, a.* From X$dbgalertext a) Aa
                                                             *
ERROR at line 2:
ORA-00942: table or view does not exist

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
PL/SQL Release 11.2.0.1.0 - Production
CORE    11.2.0.1.0      Production
TNS for 32-bit Windows: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production