Nested loop vs Hash Join

Question

Hi,
Both the querys are returning same results, but in my first query hash join and second query nested loop . How ? PLs explain
select *
from emp a,dept b
where a.deptno=b.deptno and b.deptno>20;
6 rows
Plan hash value: 4102772462
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |         |     6 |   348 |     6  (17)| 00:00:01 |
|*  1 |  HASH JOIN                   |         |     6 |   348 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     3 |    60 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | PK_DEPT |     3 |       |     1   (0)| 00:00:01 |
|*  4 |   TABLE ACCESS FULL          | EMP     |     7 |   266 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - access("A"."DEPTNO"="B"."DEPTNO")
   3 - access("B"."DEPTNO">20)
   4 - filter("A"."DEPTNO">20)
select *
from emp a,dept b
where a.deptno=b.deptno and b.deptno=30;  
6 rows
Plan hash value: 568005898
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |         |     5 |   290 |     4   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |         |     5 |   290 |     4   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     1 |    20 |     1   (0)| 00:00:01 |
|*  3 |    INDEX UNIQUE SCAN         | PK_DEPT |     1 |       |     0   (0)| 00:00:01 |
|*  4 |   TABLE ACCESS FULL          | EMP     |     5 |   190 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   3 - access("B"."DEPTNO"=30)
   4 - filter("A"."DEPTNO"=30)

Answer 1

Hi,
Unless specifically requested, Oracle picks the best execution plan based on estimates of table sizes, column selectivity and many other variables. Even though Oracle does its best to have the estimates as accurate as possible, they are frequently different, and in some cases quite different, from the actual values.
In the first query, Oracle estimated that the predicate “ b.deptno>20” would limit the number of records to 6, and based on that it decided the use Hash Join.
In the second query, Oracle estimated that the predicate “b.deptno=30” would limit the number of records to 5, and based on that it decided the use Nested Loops Join.
The fact that the actual number of records is the same is irrelevant because Oracle used the estimate, rather the actual number of records to pick the best plan.
HTH,
Iordan
Iotzov