How to find the largest segments in Oracle

The biggest segments in the database, tables, indexes, and LOBs together, ranked by size. The top 30 is usually enough to explain where the space went.

SELECT * FROM (
  SELECT owner,
         segment_name,
         segment_type,
         tablespace_name,
         ROUND(bytes/1024/1024/1024, 2) AS gb
  FROM   dba_segments
  ORDER  BY bytes DESC
)
WHERE ROWNUM <= 30;