solution for used_memory desc

SELECT s.username,
SUBSTR(s.sid,1,5)sid,
p.spid,
s.SERIAL#,
logon_time,
DECODE(TRUNC(SYSDATE - LOGON_TIME), 0, NULL, TRUNC(SYSDATE - LOGON_TIME)
|| ' Days' || ' + ') ||
TO_CHAR(TO_DATE(TRUNC(MOD(SYSDATE-LOGON_TIME,1) * 86400), 'SSSSS'),
'HH24:MI:SS') LOGON,
SUBSTR(s.program,1,22) program ,
MODULE ,
s.process pid_remote,
s.status,
ROUND(pga_used_mem/1024/1024) used_memory,
ROUND(pga_alloc_mem/1024/1024) alocation_memory,
ROUND(pga_freeable_mem/1024/1024) free_memory,
ROUND(pga_max_mem/1024/1024) maximum_memory
FROM v$session s,v$process p
WHERE p.addr=s.paddr
ORDER BY used_memory desc;