Explain for Explain plan

Question

Hi,
can anybody point me to very details of this Plan, specifically how to understand syntax in "3-filter..." statement marked in red.
This about cross join. I expect to see <> (not equal sign) between those columns but there is nothing:
Also actual results is 14 rows, but I don't see this number anywhere in the plan, only "12", should be any correlation between plan and result numbers?
T1 (col_1 number):
1
2
3
4
T2 (col_1 number):
3
4
5
6
SQL> explain plan for  select t1.col_1 from t1, t2 where t1.col_1 != t2.col_1;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
Plan hash value: 1967407726
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |    12 |   312 |     6   (0)| 00:00:01 |
|   1 |  NESTED LOOPS      |      |    12 |   312 |     6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     4 |    52 |     2   (0)| 00:00:01 |
|*  3 |   TABLE ACCESS FULL| T2   |     3 |    39 |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   3 - filter("T1"."COL_1""T2"."COL_1")          ---/*XXX
Note
   - dynamic sampling used for this statement

Answer 1

the cost based optimizer uses internal statistics to determine the number of rows an operation will return. For the simple query in your example these estimates would be quite exact if the statistics were adequate - but they are not since we see the dynamic sampling note that indicates missing statistics: http://blogs.oracle.com/optimizer/entry/dynamic_sampling_and_its_impact_on_the_optimizer
-- without statistics
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |    12 |   312 |    13   (0)| 00:00:01 |
|   1 |  NESTED LOOPS      |      |    12 |   312 |    13   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     4 |    52 |     4   (0)| 00:00:01 |
|*  3 |   TABLE ACCESS FULL| T2   |     3 |    39 |     2   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   3 - filter("T1"."ID"<>"T2"."ID")
Note
   - dynamic sampling used for this statement (level=2)
-- gather statistics
exec dbms_stats.gather_table_stats(user, 'T1')
exec dbms_stats.gather_table_stats(user, 'T2')
-- with statistics (we see the 4 * 4 = 16 rows we expect)
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |    16 |    96 |    13   (0)| 00:00:01 |
|   1 |  NESTED LOOPS      |      |    16 |    96 |    13   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     4 |    12 |     4   (0)| 00:00:01 |
|*  3 |   TABLE ACCESS FULL| T2   |     4 |    12 |     2   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   3 - filter("T1"."ID"<>"T2"."ID")The filter says that the results of the full scan in Operation 3 are filtered according to the given condition. If we had indexes we would perhaps see an access predicate.
Regards
Martin Preiss