"order by " clause confuses CBO

The following simple join retrieves all rows very quick and the execution plan looks fine.

SQL> select docnum from ojs_main where docnum in (select unique docnum from ojs_de where contains(inhalt,'Tokio or tokyo')>0);

4027 rows selected.

Execution Plan

   0      SELECT STATEMENT Optimizer=HINT: ALL_ROWS (Cost=2183 Card=75          
          18554 Bytes=11488350512)                                              
                                                                                
   1    0   NESTED LOOPS (Cost=2183 Card=7518554 Bytes=11488350512)             
   2    1     TABLE ACCESS (BY GLOBAL INDEX ROWID) OF 'OJS_DE' (Cost=2          
          6 Card=2156 Bytes=3266340)                                            
                                                                                
   3    2       DOMAIN INDEX OF 'OJS_DE_IDX' (Cost=24)                          
   4    1     INDEX (UNIQUE SCAN) OF 'OJS_MAIN_PK' (UNIQUE) (Cost=1 Ca          
          rd=348727 Bytes=4533451)                                              

When I add an "order by" clause Oracle 8i need much more longer than the same query above and the execution is a complete other one. docnum is of type NUMBER and is the primary key in both tables. All tables and indexes are fully analyzied. I also tried to replace the query below by a join over both tables but without any success. I received the same execution plan. Also working with hints did not work. Any idea how to get this query optimized ?

SQL> select docnum from ojs_main where docnum in (select unique docnum from ojs_de where contains(inhalt,'Tokio or tokyo')>0) order by docnum;

4027 rows selected.

Execution Plan

   0      SELECT STATEMENT Optimizer=HINT: ALL_ROWS (Cost=2431 Card=75          
          18554 Bytes=11488350512)                                              
                                                                                
   1    0   MERGE JOIN (Cost=2431 Card=7518554 Bytes=11488350512)               
   2    1     INDEX (FULL SCAN) OF 'OJS_MAIN_PK' (UNIQUE) (Cost=1440 C          
          ard=348727 Bytes=4533451)                                             
                                                                                
   3    1     SORT (JOIN) (Cost=965 Card=2156 Bytes=3266340)                    
   4    3       TABLE ACCESS (BY GLOBAL INDEX ROWID) OF 'OJS_DE' (Cost          
          =26 Card=2156 Bytes=3266340)                                          
                                                                                
   5    4         DOMAIN INDEX OF 'OJS_DE_IDX' (Cost=24 Card=2156)              





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