如何使用ordered提示改变SQL执行计划

ORDERED提示强制Oracle按照From子句中表出现的顺序进行表连接。

通过ordered提示，可以避免CBO SQL解析过程中的表连接评估，从而避免Oracle产生错误的执行计划，或者强制Oracle按照我们指定的方式执行。

在很多时候，当我们清楚地了解数据结构和数据分布之后，就可以通过ORDERED提示来提高SQL性能。

通过以下例子我们来说明一下Ordered提示的作用.

1.不加Hints时SQL的执行计划

SQL> set autotrace trace explain
SQL>  SELECT COUNT (*)
  2    FROM t_small, t_max, t_middle
  3   WHERE t_small.object_id = t_middle.object_id
  4   AND t_middle.object_id = t_max.object_id;
Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=194 Card=1 Bytes=12)
   1    0   SORT (AGGREGATE)
   2    1     HASH JOIN (Cost=194 Card=400 Bytes=4800)
   3    2       HASH JOIN (Cost=42 Card=100 Bytes=800)
   4    3         TABLE ACCESS (FULL) OF 'T_SMALL' (Cost=2 Card=100 Bytes=400)
   5    3         TABLE ACCESS (FULL) OF 'T_MIDDLE' (Cost=39 Card=28447 Bytes=113788)
   6    2       TABLE ACCESS (FULL) OF 'T_MAX' (Cost=151 Card=113792 Bytes=455168)

我们可以通过10053事件跟踪一下该SQL的解析:

SQL> alter session set events='10053 trace name context forever,level 1';
Session altered.
SQL> explain plan for
  2  SELECT COUNT (*)
  3    FROM t_small, t_max, t_middle
  4  WHERE t_small.object_id = t_middle.object_id
  5  AND t_middle.object_id = t_max.object_id;    
Explained.

查看Trace文件可以看到,Oracle需要进行3! (6)次表连接顺序的评估:

bash-2.03$ cat testora9_ora_10862.trc |grep "Join order"
Join order[1]: T_SMALL [T_SMALL] T_MIDDLE [T_MIDDLE] T_MAX [T_MAX] 
Join order[2]: T_SMALL [T_SMALL] T_MAX [T_MAX] T_MIDDLE [T_MIDDLE] 
Join order[3]: T_MIDDLE [T_MIDDLE] T_SMALL [T_SMALL] T_MAX [T_MAX] 
Join order[4]: T_MIDDLE [T_MIDDLE] T_MAX [T_MAX] T_SMALL [T_SMALL] 
Join order[5]: T_MAX [T_MAX] T_SMALL [T_SMALL] T_MIDDLE [T_MIDDLE] 
Join order[6]: T_MAX [T_MAX] T_MIDDLE [T_MIDDLE] T_SMALL [T_SMALL]

2.当我们使用Ordered提示之后

SQL的执行计划如下(from子句后的表顺序作了调整):

SQL> SELECT /*+ ordered */ COUNT (*)
  2    FROM t_middle, t_small, t_max
  3  WHERE t_small.object_id = t_middle.object_id
  4  AND t_middle.object_id = t_max.object_id;  
Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=197 Card=1 Bytes=12)
   1    0   SORT (AGGREGATE)
   2    1     HASH JOIN (Cost=197 Card=400 Bytes=4800)
   3    2       HASH JOIN (Cost=45 Card=100 Bytes=800)
   4    3         TABLE ACCESS (FULL) OF 'T_MIDDLE' (Cost=39 Card=28447 Bytes=113788)
   5    3         TABLE ACCESS (FULL) OF 'T_SMALL' (Cost=2 Card=100 Bytes=400)
   6    2       TABLE ACCESS (FULL) OF 'T_MAX' (Cost=151 Card=113792 Bytes=455168)

再看10053的跟踪Trace文件:

bash-2.03$ grep "Join order" testora9_ora_10918.trc
Join order[1]: T_MIDDLE [T_MIDDLE] T_SMALL [T_SMALL] T_MAX [T_MAX]

Oracle只需要按照表在From子句中的出现顺序进行连接,从而按照我们的意图进行解析或执行.

这就是Ordered提示的基本作用,本例只是一个示范说明,后者的执行计划使得Cost激增,在实际应用中,我们当然是不希望看到此类增长的.