Re: Postgres 9.0 has a bias against indexes

On 1/27/2011 3:10 PM, Igor Neyman wrote:
>
> Mladen,
>
> I don't think, this is exclusive Postgres feature.
> I'm pretty sure, Oracle optimizer will do "TABLE ACCESS (FULL)" instead
> of using index on 14-row table either.
>
> Regards,
> Igor Neyman

Well, lets' see:

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
PL/SQL Release 11.2.0.2.0 - Production
CORE 11.2.0.2.0 Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production

Elapsed: 00:00:00.00
SQL> set autotrace on explain
SQL> with e(empno,ename,mgr,bossname,lev) as (
2 select empno,ename,mgr,NULL,0 from emp where empno=7839
3 union all
4 select emp.empno,emp.ename,emp.mgr,e.ename,e.lev+1
5 from emp,e
6 where emp.mgr=e.empno)
7 select * from e
8 /

EMPNO ENAME MGR BOSSNAME LEV
---------- ---------- ---------- ---------- ----------
7839 KING 0
7566 JONES 7839 KING 1
7698 BLAKE 7839 KING 1
7782 CLARK 7839 KING 1
7499 ALLEN 7698 BLAKE 2
7521 WARD 7698 BLAKE 2
7654 MARTIN 7698 BLAKE 2
7788 SCOTT 7566 JONES 2
7844 TURNER 7698 BLAKE 2
7900 JAMES 7698 BLAKE 2
7902 FORD 7566 JONES 2

EMPNO ENAME MGR BOSSNAME LEV
---------- ---------- ---------- ---------- ----------
7934 MILLER 7782 CLARK 2
7369 SMITH 7902 FORD 3
7876 ADAMS 7788 SCOTT 3

14 rows selected.

Elapsed: 00:00:00.01

Execution Plan
----------------------------------------------------------
Plan hash value: 2925328376

--------------------------------------------------------------------------------
--------------------

| Id | Operation | Name | Rows |
Bytes | Cos
t (%CPU)| Time |

--------------------------------------------------------------------------------
--------------------

| 0 | SELECT STATEMENT | | 15 | 795 |
6 (17)| 00:00:56 |

| 1 | VIEW | | 15 | 795 |
6 (17)| 00:00:56 |

| 2 | UNION ALL (RECURSIVE WITH) BREADTH FIRST| | | |
| |

| 3 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 24 |
1 (0)| 00:00:11 |

|* 4 | INDEX UNIQUE SCAN | PK_EMP | 1 | |
0 (0)| 00:00:01 |

|* 5 | HASH JOIN | | 14 | 798 |
5 (20)| 00:00:46 |

| 6 | RECURSIVE WITH PUMP | | | |
| |

| 7 | TABLE ACCESS FULL | EMP | 14 | 336 |
3 (0)| 00:00:31 |

--------------------------------------------------------------------------------
--------------------

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

4 - access("EMPNO"=7839)
5 - access("EMP"."MGR"="E"."EMPNO")

Note
-----
- SQL plan baseline "SQL_PLAN_1tmxjj25531vff51d791e" used for this
statement

SQL> spool off

There is INDEX UNIQUE SCAN PK_EMP. Oracle will use an index.

--
Mladen Gogala
Sr. Oracle DBA
1500 Broadway
New York, NY 10036
(212) 329-5251
www.vmsinfo.com