Oracle官方文档：Oracle Database SQL Language Reference
1、ordered hint
2、leading hint
3、use_nl
1、ordered hint
/*+ ORDERED */
The ORDERED hint instructs Oracleto join tables in the order in which they appear in the FROM clause.Oracle recommends that you use the LEADING hint, which is more versatile than the ORDERED hint.
When you omit the ORDERED hint from a SQL statement requiring a join, the optimizer chooses the order in which to join the tables. You might want to use the ORDERED hint to specify a join order if you know something that the optimizer does not know about the number of rows selected from each table. Such information lets you choose an inner and outer table better than the optimizer could.
2、leading hint
/*+ LEADING ( [ @ queryblock ] tablespec [ tablespec ]... ) */
The LEADING hint instructs the optimizerto use the specified set of tables as the prefix in the execution plan. This hint is more versatile than the ORDERED hint.
The LEADING hint is ignored if the tables specified cannot be joined first in the order specified because of dependencies in the join graph. If you specify two or more conflicting LEADING hints, then all of them are ignored. If you specify the ORDERED hint, it overrides all LEADING hints.
3、use_nl hint
The USE_NL hint instructs the optimizer to join each specified table to another row source with a nested loops join, using the pecified table as the inner table.
Use of the USE_NL and USE_MERGE hints is recommended with the LEADING and ORDERED hints. The optimizer uses those hints when the referenced table is forced tobe the inner table of a join. The hints are ignored if the referenced table is the outer table.
--USE_NL强制把referenced table作为inner table。如果referenced table 为outer table，则此hint被忽略（即不管用）--个人觉得这句话是废话。
--实例1：
--/*+ ordered */ hint实例,表BASOPT上有optid列上的索引PK_BASOPT
SQL> set autot trace exp
--不用/*+ ordered */hint，BASOPTUSER作为驱动表，用BASOPTUSER去连接BASOPT表
SQL> select optname,userid from basopt a,basoptuser b where a.optid = b.optid and b.userid = 1;
执行计划
----------------------------------------------------------
Plan hash value: 922486247
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| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 19 | 4 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | | | | |
| 2 | NESTED LOOPS | | 1 | 19 | 4 (0)| 00:00:01 |
|* 3 | TABLE ACCESS FULL | BASOPTUSER | 1 | 8 | 3 (0)| 00:00:01 |
|* 4 | INDEX UNIQUE SCAN | PK_BASOPT | 1 | | 0 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| BASOPT | 1 | 11 | 1 (0)| 00:00:01 |
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Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("B"."USERID"=1)
4 - access("A"."OPTID"="B"."OPTID")
--用/*+ ordered */hint 来指定按照from后边表的顺序来连接表，用BASOPT去连接BASOPTUSER表，此时优化器选择了另一种链接方法：MERGE JOIN
SQL> select /*+ ordered */ optname,userid from basopt a,basoptuser b where a.optid = b.optid and b.userid = 1;
执行计划
----------------------------------------------------------
Plan hash value: 2164325570
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Ti