SELECT segment_type, 
      SUM(bytes) bytes,
      CUME_DIST() OVER (ORDER BY SUM(bytes)) bytes_percentile
      FROM sys.dba_segments
      GROUP BY segment_type;
--
SELECT * 
FROM
(SELECT segment_type, 
SUM(bytes) bytes,
CUME_DIST() OVER (ORDER BY SUM(bytes)) bytes_percentile
FROM sys.dba_segments
GROUP BY segment_type)
WHERE bytes_percentile >= 0.5;
--
SELECT * 
FROM
(
SELECT owner,
        SUM(bytes) bytes,
        RANK() OVER(ORDER BY SUM(bytes) DESC) bytes_rank
FROM sys.dba_segments
WHERE segment_type IN
      (SELECT segment_type
        FROM
           (SELECT segment_type, 
                SUM(bytes) bytes,
                CUME_DIST() OVER (ORDER BY SUM(bytes)) bytes_percentile
                FROM sys.dba_segments
                GROUP BY segment_type)
        WHERE bytes_percentile >= 0.5)
GROUP BY owner
)
WHERE bytes_rank <=5;