Index usage depends on columns selected

Question

Hi, somehow cannot understand why index is not used, please help.
in Oracle Database 11g Release 11.2.0.3.0 - 64bit Production
1.     Included only indexed column and got a perfect plan
explain plan for2 select s.x_cnt
3 from reported_summary s
4 where s.x_cnt>0;
PLAN_TABLE_OUTPUT
Plan hash value: 2674489506
| Id | Operation | Name | Cost (%CPU)|
| 0 | SELECT STATEMENT | | 306 (8)|
|* 1 | INDEX FAST FULL SCAN| S_NUI01 | 306 (8)|
Predicate Information (identified by operation id):
1 - filter("s"."x_CNT">0)
2.     Included some other column and got TABLE ACCESS FULL
explain plan for2 select s.x_cnt,s.ru_id
3 from reported_summary s
4 where s.x_cnt>0;
PLAN_TABLE_OUTPUT
Plan hash value: 2142873335
| Id | Operation | Name | Cost (%CPU)|
| 0 | SELECT STATEMENT | | 2421 (3)|
|* 1 | TABLE ACCESS FULL| REPORTED_SUMMARY | 2421 (3)|
Predicate Information (identified by operation id):
1 - filter("s"."x_CNT">0)
3.     Included all other columns and got TABLE ACCESS FULL as well
explain plan for2 select s.x_cnt,s.*
3 from reported_summary s
4 where s.x_cnt>0;
PLAN_TABLE_OUTPUT
Plan hash value: 2142873335
| Id | Operation | Name | Cost (%CPU)|
| 0 | SELECT STATEMENT | | 2421 (3)|
|* 1 | TABLE ACCESS FULL| REPORTED_SUMMARY | 2421 (3)|
Predicate Information (identified by operation id):
1 - filter("s"."x_CNT">0)
Thanks a lot

Answer 1

Thanks all, just to clarify
"select s.x_cnt from reported_summary s..." is using index;
"select 'Y' y from reported_summary s..." is using index;
"select <any ohter column> from reported_summary s..." is causing TABLE ACCESS FULL;
no differences in where clause, no order by at all
the index script is
CREATE INDEX S_NUI01 ON REPORTED_SUMMARY (OFFSET_CNT) TABLESPACE X_INDEXES;
jgarry, thanks for the answer, the only problem that I have clone of the database (32 bit) with same CBO parameters
where it does not behave this way:
explain plan for2 select s.offset_cnt
3 from reported_summary s
4 where s.offset_cnt>0;
PLAN_TABLE_OUTPUT
Plan hash value: 2359470296
| Id | Operation | Name | Cost (%CPU)|
| 0 | SELECT STATEMENT | | 9 (0)|
|* 1 | INDEX RANGE SCAN| S_NUI01 | 9 (0)|
Predicate Information (identified by operation id):
1 - access("s"."OFFSET_CNT">0)
explain plan for2 select s.offset_cnt,s.ru_id
3 from reported_summary s
4 where s.offset_cnt>0;
PLAN_TABLE_OUTPUT
Plan hash value: 3732627180
| Id | Operation | Name | Cost (%CPU)|
| 0 | SELECT STATEMENT | | 67 (0)|
| 1 | TABLE ACCESS BY INDEX ROWID| REPORTED_SUMMARY | 67 (0)|
|* 2 | INDEX RANGE SCAN | S_NUI01 | 9 (0)|
Predicate Information (identified by operation id):
2 - access("s"."OFFSET_CNT">0)
Thanks again.