Index monitoring - v$object_usage returns no rows

Cause: v$object_usage view is specific to the currently connected user.

Solution:  Either connect as the object owner, or create the following view as sys

create or replace view V$ALL_OBJECT_USAGE
  (OWNER,
   INDEX_NAME,
   TABLE_NAME,
   MONITORING,
   USED,
   START_MONITORING,
   END_MONITORING
  )
  as
  select u.name,
         io.name,
         t.name,
         decode(bitand(i.flags, 65536), 0, 'NO', 'YES'),
         decode(bitand(ou.flags, 1), 0, 'NO', 'YES'),
         ou.start_monitoring, 
         ou.end_monitoring
    from sys.user$ u,
         sys.obj$ io, 
         sys.obj$ t, 
         sys.ind$ i, 
         sys.object_usage ou
   where i.obj# = ou.obj#
     and io.obj# = ou.obj#
     and t.obj# = i.bo#
     and u.user# = io.owner#
/

MEMORY USED BY SQL STATEMENT

select machine,to_char(SQL_EXEC_START,'DD-MON-YY HH24:MI:SS')            runtime,
       pga_allocated 
  from dba_hist_active_sess_history 
 where sql_id = '&sql_id';


EXAMPLE OUTPUT:

MACHINE                   RUNTIME                 PGA_ALLOCATED

------------------------- -------------------- ----------------
MYPCNAME                  13-JUL-16 16:18:40        807,153,664
MYPCNAME                  13-JUL-16 16:18:40        329,003,008
MYPCNAME                  13-JUL-16 16:18:40        274,280,448
MYPCNAME                  13-JUL-16 16:18:40        215,887,872
MYPCNAME                  13-JUL-16 16:18:40        204,615,680