Query 1 shows less consistent gets but more cost than Query 2..

Question

Hi ,
SQL> select dname from scott.dept where deptno not in (select deptno from scott.emp)
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Plan hash value: 3547749009
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |    1 |    22 | 4 (0)| 00:00:01 |
|*  1 |  FILTER            |      |       |       |            |          |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |    88 | 2 (0)| 00:00:01 |
|*  3 |   TABLE ACCESS FULL| EMP  |    11 |   143 |  2 (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter( NOT EXISTS (SELECT /*+ */ 0 FROM "SCOTT"."EMP" "EMP"
              WHERE LNNVL("DEPTNO"<>:B1)))
   3 - filter(LNNVL("DEPTNO"<>:B1))
Note
   - dynamic sampling used for this statement
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          0  recursive calls
          0  db block gets
         15 consistent gets
          0  physical reads
          0  redo size
        416  bytes sent via SQL*Net to client
        384  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL>
SQL> select dname from scott.dept,scott.emp where dept.deptno=emp.deptno(+)
  2    and emp.rowid is null;
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Plan hash value: 2146709594
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT    |      |   12 |   564 | 5 (20)| 00:00:01 |
|*  1 |  FILTER             |      |       |       |            |          |
|*  2 |   HASH JOIN OUTER   |      |    12 |   564 | 5 (20)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| DEPT |     4 |    88 | 2 (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| EMP  |    12 |   300 | 2 (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("EMP".ROWID IS NULL)
   2 - access("DEPT"."DEPTNO"="EMP"."DEPTNO"(+))
Note
   - dynamic sampling used for this statement
ÓôáôéóôéêÜ
          0  recursive calls
          0  db block gets
          6 consistent gets
          0  physical reads
          0  redo size
        416  bytes sent via SQL*Net to client
        384  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedI have two questions:
1) which one is preferable.... the first which is less costy to the system or the second which causes less consistent gets to the system and so is considered to be more scalable..????
2)Whereas the number of rows returned by both queries is 1.. why the is difference in the underlined values in the two queries (values 1 and 12 respectively)?
I use Oracle10g v.2
Thanks.. a lot
Sim

Answer 1

The less logical I/O's the better.
So always do it like your query 2 (btw. your title is the wrong way)
Your example is probably flawed. If I try it in SQL*Plus I get correct results:
SQL> get t
  1* select dname from dept where deptno not in (select deptno from emp)
SQL> /
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=6 Card=3 Bytes=39)
   1    0   FILTER
   2    1     TABLE ACCESS (FULL) OF 'DEPT' (TABLE) (Cost=2 Card=4 Bytes=52)
   3    1     TABLE ACCESS (FULL) OF 'EMP' (TABLE) (Cost=2 Card=1 Bytes=3)
Statistics
          0  recursive calls
          0  db block gets
         15  consistent gets
          0  physical reads
          0  redo size
        537  bytes sent via SQL*Net to client
        660  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> get tt
  1  select dname from dept,emp where dept.deptno=emp.deptno(+)
  2* and emp.rowid is null
SQL> /
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=5 Card=14 Bytes=322)
   1    0   FILTER
   2    1     HASH JOIN (OUTER) (Cost=5 Card=14 Bytes=322)
   3    2       TABLE ACCESS (FULL) OF 'DEPT' (TABLE) (Cost=2 Card=4 Bytes=52)
   4    2       TABLE ACCESS (FULL) OF 'EMP' (TABLE) (Cost=2 Card=14 Bytes=140)
Statistics
          0  recursive calls
          0  db block gets
          6  consistent gets
          0  physical reads
          0  redo size
        537  bytes sent via SQL*Net to client
        660  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> I'm wondering for instance why you have there 11 rows in emp for query 1 (should be only 1 row) and why you have only 12 rows in query 2 (should be 14 rows)