使用hint优化Oracle的执行计划

背景：

某表忽然出现查询非常缓慢的情况，cost 100+ 秒以上；严重影响生产。

原SQL：

explain plan for 
select * from (
select ID id,RET_NO retNo, FROM_SYS fromSy, TO_SYS toSys, COMMAND_code commandCode, COMMAND, STATUS, 
EXT_code, ORIGN_code orignCode,error_message errorMessage, RE_F, RET_MSG retMsg 
from interface_table where ((command_code in('AASSS') 
			and  status in('F','E') and (re_f = 'N') and FROM_SYS = 'MEE')
			or (COMMAND_code in('XXXX','XXXX9') and FROM_SYS = 'EXT' and RE_F = 'N')
			) and MOD(id, 1) = 0  order by id) where rownum <= 100  ;

SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY('PLAN_TABLE'));

Plan hash value: 1871549687
 
----------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name               | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                    |    99 |   382K|   637   (1)| 00:00:08 |
|*  1 |  COUNT STOPKEY                |                    |       |       |            |          |
|   2 |   VIEW                        |                    |   100 |   386K|   637   (1)| 00:00:08 |
|*  3 |    TABLE ACCESS BY INDEX ROWID| INTERFACE_TABLE    |   355 | 55735 |   637   (1)| 00:00:08 |
|*  4 |     INDEX FULL SCAN           | PK_INTERFACE_TABLE |  1439 |       |   280   (2)| 00:00:04 |
----------------------------------------------------------------------------------------------------

优化后的SQL：

explain plan for 
select * from (
select /*+ index(INT_TABLE IX_INT_TABLE_2)*/ ID id,RET_NO retNo, FROM_SYS fromSy, TO_SYS toSys, COMMAND_code commandCode, COMMAND, STATUS, 
EXT_code, ORIGN_code orignCode,error_message errorMessage, RE_F, RET_MSG retMsg 
from interface_table where ((command_code in('AASSS') 
			and  status in('F','E') and (re_f = 'N') and FROM_SYS = 'MEE')
			or (COMMAND_code in('XXXX','XXXX9') and FROM_SYS = 'EXT' and RE_F = 'N')
			) and MOD(id, 1) = 0 order by id) where rownum <= 100  ;

SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY('PLAN_TABLE'));

Plan hash value: 3625182869
 
--------------------------------------------------------------------------------------------------------
| Id  | Operation                       | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                |                      |    99 |   382K| 19105   (1)| 00:03:50 |
|*  1 |  COUNT STOPKEY                  |                      |       |       |            |          |
|   2 |   VIEW                          |                      |   356 |  1376K| 19105   (1)| 00:03:50 |
|*  3 |    SORT ORDER BY STOPKEY        |                      |   356 | 55892 | 19105   (1)| 00:03:50 |
|   4 |     CONCATENATION               |                      |       |       |            |          |
|*  5 |      TABLE ACCESS BY INDEX ROWID| INTERFACE_TABLE      |    69 | 10833 |  9552   (1)| 00:01:55 |
|*  6 |       INDEX RANGE SCAN          | IX_INTERFACE_TABLE_2 | 77145 |       |    99   (0)| 00:00:02 |
|*  7 |      TABLE ACCESS BY INDEX ROWID| INTERFACE_TABLE      |   287 | 45059 |  9552   (1)| 00:01:55 |
|*  8 |       INDEX RANGE SCAN          | IX_INTERFACE_TABLE_2 | 77145 |       |    99   (0)| 00:00:02 |
--------------------------------------------------------------------------------------------------------

查看执行计划，原来是使用 full scan - 当数据量大时非常慢；优化后oracle优先走range scan，hint 的 index 是未处理标识字段的索引，正常情况下这个数据集合相对较小--------所以可以达到优化目的。

具体情况具体分析，我们必须要看实际的表存的业务数据，分析其业务关系找到最小业务集合；后者要看懂执行计划，根据rows, bytes, cost, time 找到最优项目。这个分析顺序不能倒置。

问题：为何使用 rownum 后，oracle执行计划会走full scan？

使用hint优化Oracle的执行计划,布布扣,bubuko.com