Search notes:

Oracle: X$DIAG_ALERT_EXT

select
   originating_timestamp,
   normalized_timestamp,
   message_group,
   message_text,
   operation_id,
   cause_text,
   component_id,
   message_type,
   message_level,
   message_id
from
   x$diag_alert_ext
order by
   indx desc;

See also

x$dbgalertext
fixed tables

Index