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 ;查看其执行计划：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 ;
查看其执行计划：
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 | --------------------------------------------------------------------------------------------------------
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？
转：如何看懂执行计划：http://jadethao.iteye.com/blog/1613943