SELECT trunc(b.begin_interval_time),
round(sum(GREATEST(
((SELECT d.value FROM dba_hist_sysstat d WHERE d.snap_id=a.snap_id+1 
AND d.instance_number=a.instance_number 
AND d.stat_name 
= 'flashback log write bytes') - a.value),0)/1024/1024))
 FROM dba_hist_sysstat a,dba_hist_snapshot b 
WHERE a.snap_id=b.snap_id AND a.instance_number=b.instance_number
 AND a.stat_name = 'flashback log write bytes' 
GROUP BY  trunc(b.begin_interval_time) ORDER BY 1 ;