10g之前的系统，修改这个参数：

SQL> alter system set optimizer_mode=first_rows scope=both;
alter system set optimizer_mode=first_rows scope=both
                                           *
ERROR at line 1:
ORA-02096: specified initialization parameter is not modifiable with this
option

SQL> alter system set optimizer_mode=first_rows scope=memory;
alter system set optimizer_mode=first_rows scope=memory

                                          *
ERROR at line 1:
ORA-02096: specified initialization parameter is not modifiable with this
option

#指定的初始化参数不可以修改

SQL> alter system set optimizer_mode=first_rows scope=spfile
  2  /

System altered.

SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
PL/SQL Release 9.2.0.4.0 - Production
CORE    9.2.0.3.0       Production
TNS for Linux: Version 9.2.0.4.0 - Production
NLSRTL Version 9.2.0.4.0 - Production

10g 可以直接修改这个参数，两种模式都可生效

SQL> alter system set optimizer_mode=first_rows_100 scope=both;

System altered.


SQL> show parameters  optimizer_mode;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
optimizer_mode                       string      FIRST_ROWS_100


SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - Prod
PL/SQL Release 10.1.0.2.0 - Production
CORE    10.1.0.2.0      Production
TNS for Linux: Version 10.1.0.2.0 - Production
NLSRTL Version 10.1.0.2.0 - Production

SQL>

这种变化从一定程度上来说，大大提高了可用性

呵呵，技术只会越来越先进地

奇怪，9i的黩认是choose,
10g黩认是all_rows