Is this a bug with grouping sets?

Question

Version is Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
SQL> with data as
  2  (select rownum r, 'X' x, 1 n from dual connect by rownum <= 10)
  3  select x
  4       , sum(n) s
  5       , (select sum(1) from dual where dummy = x) t
  6  from data
  7  group by grouping sets (x, ())
  8  /
X          S          T
X         10          1
          10
SQL>  Shouldn't the superaggregate row have a 1 in the T column?

Answer 1

Here's your QUERY with the addition of the GROUPING() function:
SQL> WITH data AS
  2  (
  3     SELECT rownum r
  4          , 'X'    x
  5          , 1      n
  6     FROM   dual
  7     CONNECT BY rownum <= 10
  8  )
  9  SELECT GROUPING(x)
10       , x
11       , sum(n) s
12       , (SELECT sum(1) FROM dual WHERE dummy = x) t
13  FROM   data
14  GROUP BY GROUPING SETS (x, ())
15  ;
GROUPING(X) X          S          T
          0 X         10          1
          1           10In this case the value for X is null because it's a subtotal row. If you look at the execution plan for this statement:
PLAN_TABLE_OUTPUT
SQL_ID  gumphwvgumqc4, child number 0
WITH data AS (  SELECT rownum r       , 'X'    x       , 1      n  FROM
  dual  CONNECT BY rownum <= 10 ) SELECT /*+gather_plan_statistics*/ x
    , sum(n) s      , (SELECT sum(1) FROM dual WHERE dummy = x) t FROM
data GROUP BY GROUPING SETS (x, ())
Plan hash value: 1718326399
| Id  | Operation                       | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
|   0 | SELECT STATEMENT                |      |      1 |        |      2 |00:00:00.01 |       0 |    |  |          |
|   1 |  SORT AGGREGATE                 |      |      2 |      1 |      2 |00:00:00.01 |       4 |    |  |          |
|*  2 |   TABLE ACCESS FULL             | DUAL |      2 |      1 |      1 |00:00:00.01 |       4 |    |  |          |
|   3 |  SORT GROUP BY ROLLUP           |      |      1 |      1 |      2 |00:00:00.01 |       0 |  2048 |  2048 | 2048  (0)|
|   4 |   VIEW                          |      |      1 |      1 |     10 |00:00:00.01 |       0 |    |  |          |
|   5 |    COUNT                        |      |      1 |        |     10 |00:00:00.01 |       0 |    |  |          |
|   6 |     CONNECT BY WITHOUT FILTERING|      |      1 |        |     10 |00:00:00.01 |       0 |    |  |          |
|   7 |      FAST DUAL                  |      |      1 |      1 |      1 |00:00:00.01 |       0 |    |  |          |
Predicate Information (identified by operation id):
   2 - filter("DUMMY"=:B1)The value for X is being bound to the subquery. If X is null this won't return any rows (I'm sure you knew that :) ). I would say this is expected albeit confusing behavior. You get the same results if you use CUBE or ROLLUP as well.