Oracle Hash Scan

The Oracle Optimizer considers a hash scan when a query accesses a table in a hash cluster.

In a hash cluster, all rows with the same hash value are stored in the same data block. To perform a hash scan, Oracle Database first obtains the hash value by applying a hash function to a cluster key value specified by the statement. Oracle Database then scans the data blocks containing rows with that hash value. Now in this example, in order for Oracle to do a hash scan, Oracle Database first obtains the hash value by applying a hash function to the key value 30, and then uses this hash value to scan the data blocks and retrieve the rows.

Lets see how we can do it :-

CREATE CLUSTER employees_departments_cluster
   (deptno NUMBER(2)) SIZE 8192 HASHKEYS 100;

CREATE TABLE employees2
   CLUSTER employees_departments_cluster (deptno)
   AS SELECT * FROM emp;

CREATE TABLE departments2
   CLUSTER employees_departments_cluster (deptno)
   AS SELECT * FROM dept;

You query the employees in department 30 as follows:

SQL> SELECT * FROM   employees2 WHERE  deptno = 30;

     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
      7499 ALLEN      SALESMAN        7698 20-FEB-81       1600        300         30
      7521 WARD       SALESMAN        7698 22-FEB-81       1251        500         30
      7654 MARTIN     SALESMAN        7698 28-SEP-81       1250       1400         30
      7698 BLAKE      MANAGER         7839 01-MAY-81       2850                    30
      7844 TURNER     SALESMAN        7698 08-SEP-81       1500          0         30
      7900 JAMES      CLERK           7698 03-DEC-81        950                    30

6 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1423052330

--------------------------------------------------------------------------------
| Id  | Operation         | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |            |     1 |    87 |     1   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS HASH| EMPLOYEES2 |     1 |    87 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("DEPTNO"=30)

Note
-----
   - dynamic statistics used: dynamic sampling (level=2)


Statistics
----------------------------------------------------------
         15  recursive calls
          0  db block gets
         77  consistent gets
         64  physical reads
          0  redo size
       1279  bytes sent via SQL*Net to client
        551  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          6  rows processed