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对于CBO优化器：
CBO根据统计信息选择驱动表，假如没有统计信息，则在from 子句中从左到右的顺序选择驱动表。这与RBO选择的顺序正好相反。这是英文原文(CBO determines join order from costs derived from gathered statistics. If there are no stats then CBO chooses the driving order of tables from LEFT to RIGHT in the FROM clause. This is OPPOSITE to the RBO) 。我还是没法证实这句话的正确性。不过经过验证：“如果用ordered 提示(此时肯定用CBO)，则以from 子句中按从左到右的顺序选择驱动表”这句话是正确的。实际上在CBO中，如果有统计数据(即对表与索引进行了分析)，则优化器会自动根据cost值决定采用哪种连接类型，并选择合适的驱动表，这与where子句中各个限制条件的位置没有任何关系。如果我们要改变优化器选择的连接类型或驱动表，则就需要使用hints了，具体hints的用法在后面会给予介绍。
测试：
如果我创建的3个表：
create table A(col1 number(4,0),col2 number(4,0), col4 char(30));
create table B(col1 number(4,0),col3 number(4,0), name_b char(30));
create table C(col2 number(4,0),col3 number(4,0), name_c char(30));
create index inx_col12A on a(col1,col2);
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执行查询：
select A.col4
from B, A, C
where B.col3 = 10
and A.col1 = B.col1
and A.col2 = C.col2
and C.col3 = 5;
Execution Plan
---------------------------
0 SELECT STATEMENT
Optimizer=ALL_ROWS
(Cost=3 Card=1 Bytes=110)
1 0 NESTED LOOPS
(Cost=3 Card=1 Bytes=110)
2 1 MERGE JOIN (CARTESIAN)
(Cost=2 Card=1 Bytes=52)
3 2 TABLE ACCESS (FULL) OF 'B'
(Cost=1 Card=1 Bytes=26)
4 2 SORT (JOIN)
(Cost=1 Card=1 Bytes=26)
5 4 TABLE ACCESS (FULL) OF 'C'
(Cost=1 Card=1 Bytes=26)
6 1 TABLE ACCESS (FULL) OF 'A'
(Cost=1 Card=82 Bytes=4756)
select A.col4
from B, A, C
where A.col1 = B.col1
and A.col2 = C.col2;
Execution Plan
------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS
(Cost=5 Card=55 Bytes=4620)
1 0 HASH JOIN
(Cost=5 Card=55 Bytes=4620)
2 1 HASH JOIN
(Cost=3 Card=67 Bytes=4757)
3 2 TABLE ACCESS (FULL) OF 'B'
(Cost=1 Card=82 Bytes=1066)
4 2 TABLE ACCESS (FULL) OF 'A'
(Cost=1 Card=82 Bytes=4756)
5 1 TABLE ACCESS (FULL) OF 'C'
(Cost=1 Card=82 Bytes=1066)
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将A表上的索引inx_col12A删除后：
select A.col4
from B, A, C
where A.col1 = B.col1
and A.col2 = C.col2;
Execution Plan
----------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS
(Cost=5 Card=55 Bytes=4620)
1 0 HASH JOIN
(Cost=5 Card=55 Bytes=4620)
2 1 HASH JOIN
(Cost=3 Card=67 Bytes=4757)
3 2 TABLE ACCESS (FULL) OF 'B'
(Cost=1 Card=82 Bytes=1066)
4 2 TABLE ACCESS (FULL) OF 'A'
(Cost=1 Card=82 Bytes=4756)
5 1 TABLE ACCESS (FULL) OF 'C'
(Cost=1 Card=82 Bytes=1066)
select /*+ ORDERED */A.col4
from C, A, B
where B.col3 = 10
and A.col1 = B.col1
and A.col2 = C.col2
and C.col3 = 5;
Execution Plan
----------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS
(Cost=3 Card=1 Bytes=110)
1 0 NESTED LOOPS
(Cost=3 Card=1 Bytes=110)
2 1 NESTED LOOPS
(Cost=2 Card=1 Bytes=84)
3 2 TABLE ACCESS (FULL) OF 'C'
(Cost=1 Card=1 Bytes=26)
4 2 TABLE ACCESS (FULL) OF 'A'
(Cost=1 Card=82 Bytes=4756)
5 1 TABLE ACCESS (FULL) OF 'B'
(Cost=1 Card=1 Bytes=26)
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这个查询验证了通过ORDERED提示可以正确的提示优化器选择哪个表作为优化器。
(责任编辑：卢兆林）
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