Re: cost

From: Daniel Fink <Daniel.Fink_at_Sun.COM>
Date: Mon, 05 Apr 2004 12:40:08 -0600
Message-id: <4071A808.67996A08@sun.com>

Riyas,

As we say in the states, "Bingo!"

select e.ename, d.dname
from emp e inner join dept d
using (deptno)
where d.dname = 'RESEARCH';

Execution Plan

---

   0 SELECT STATEMENT Optimizer=CHOOSE (Cost=5 Card=5 Bytes=90)

   1 0 HASH JOIN (Cost=5 Card=5 Bytes=90)    2 1 TABLE ACCESS (FULL) OF 'DEPT' (Cost=2 Card=1 Bytes=11)

   3 1 TABLE ACCESS (FULL) OF 'EMP' (Cost=2 Card=14 Bytes=98)

alter session set optimizer_mode = all_rows;

select e.ename, d.dname
from emp e inner join dept d
using (deptno)
where d.dname = 'RESEARCH';

Execution Plan

---

   0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=5 Card=5 Bytes=90)

   1 0 HASH JOIN (Cost=5 Card=5 Bytes=90)    2 1 TABLE ACCESS (FULL) OF 'DEPT' (Cost=2 Card=1 Bytes=11)

   3 1 TABLE ACCESS (FULL) OF 'EMP' (Cost=2 Card=14 Bytes=98)

alter session set optimizer_mode = first_rows;

select e.ename, d.dname
from emp e inner join dept d
using (deptno)
where d.dname = 'RESEARCH';

Execution Plan

---

   0 SELECT STATEMENT Optimizer=FIRST_ROWS (Cost=4 Card=5 Bytes=90)

   1 0 NESTED LOOPS (Cost=4 Card=5 Bytes=90)    2 1 TABLE ACCESS (FULL) OF 'DEPT' (Cost=2 Card=1 Bytes=11)

   3 1 TABLE ACCESS (FULL) OF 'EMP' (Cost=2 Card=5 Bytes=35)

Excellent Catch!

Riyaj Shamsudeen wrote:
>
> *I think*, this is due to optimizer heuristics . First_rows cost model
> is implemented as an heuristics model and applied over the usual cost
> comparisons. I have seen optimizer choose a costlier plan because this
> heuristics was/was not applied.
> I wonder what results you would get if you have optimizer_mode set to
> first_rows in your test case ?
> Thanks
> Riyaj "Re-yas" Shamsudeen
> Certified Oracle DBA
>

---

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Received on Mon Apr 05 2004 - 13:37:04 CDT