Re: Why CBO choose wrong way?
- From: Connor McDonald <mcdonald.connor@xxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Sat, 28 May 2005 21:21:02 +0800
A similar example at http://www.oracledba.co.uk/tips/9i_first_rows.htm
hth
connor
On 5/28/05, Zhai Jingmin <zhai_jingmin@xxxxxxxxx> wrote:
> I changed optimizer_mode to 'first_rows_1',and my query can use the right=
index too!I never thought there's so much difference between 'first_rows' =
and 'first_rows_1' before this query problem,I checked the oracle manuels,f=
ound following words,please note the 'Note',hope anyone like me note this b=
ig difference.Thanks a lot for Lex's notification.
>=20
> FIRST_ROWS_n
>=20
> The optimizer uses a cost-based approach, regardless of the presence of s=
tatistics, and optimizes with a goal of best response time to return the fi=
rst n number of rows; n can equal 1, 10, 100, or 1000.
>=20
> FIRST_ROWS
>=20
> The optimizer uses a mix of cost and heuristics to find a best plan for f=
ast delivery of the first few rows.
>=20
> Note: Using heuristics sometimes leads the query optimizer to generate a =
plan with a cost that is significantly larger than the cost of a plan witho=
ut applying the heuristic. FIRST_ROWS is available for backward compatibili=
ty and plan stability; use FIRST_ROWS_n instead.
>=20
--=20
Connor McDonald
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D
email: connor_mcdonald@xxxxxxxxx
web: http://www.oracledba.co.uk
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