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。

Oracle入门教程：leading vs ordered hint 

Oracle hint 实践一列 leanding 驱动表和hash多块读取

Oracle优化常用hint语句

Oracle调优hint提示

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  ;

查看其执行计划：

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

  1. Plan hash value: 1871549687
  3. ----------------------------------------------------------------------------------------------------
  4. | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
  5. ----------------------------------------------------------------------------------------------------
  6. | 0 | SELECT STATEMENT | | 99 | 382K| 637 (1)| 00:00:08 |
  7. |* 1 | COUNT STOPKEY | | | | | |
  8. | 2 | VIEW | | 100 | 386K| 637 (1)| 00:00:08 |
  9. |* 3 | TABLE ACCESS BY INDEX ROWID| INTERFACE_TABLE | 355 | 55735 | 637 (1)| 00:00:08 |
  10. |* 4 | INDEX FULL SCAN | PK_INTERFACE_TABLE | 1439 | | 280 (2)| 00:00:04 |
  11. ----------------------------------------------------------------------------------------------------

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  ;

查看其执行计划：

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

  1. Plan hash value: 3625182869
  3. --------------------------------------------------------------------------------------------------------
  4. | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
  5. --------------------------------------------------------------------------------------------------------
  6. | 0 | SELECT STATEMENT | | 99 | 382K| 19105 (1)| 00:03:50 |
  7. |* 1 | COUNT STOPKEY | | | | | |
  8. | 2 | VIEW | | 356 | 1376K| 19105 (1)| 00:03:50 |
  9. |* 3 | SORT ORDER BY STOPKEY | | 356 | 55892 | 19105 (1)| 00:03:50 |
  10. | 4 | CONCATENATION | | | | | |
  11. |* 5 | TABLE ACCESS BY INDEX ROWID| INTERFACE_TABLE | 69 | 10833 | 9552 (1)| 00:01:55 |
  12. |* 6 | INDEX RANGE SCAN | IX_INTERFACE_TABLE_2 | 77145 | | 99 (0)| 00:00:02 |
  13. |* 7 | TABLE ACCESS BY INDEX ROWID| INTERFACE_TABLE | 287 | 45059 | 9552 (1)| 00:01:55 |
  14. |* 8 | INDEX RANGE SCAN | IX_INTERFACE_TABLE_2 | 77145 | | 99 (0)| 00:00:02 |
  15. --------------------------------------------------------------------------------------------------------

比较：

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？

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