关于COUNT STOPKEY的工作机制(转载)

SQL> select rownum rn ,a.* from cnmir.ew_auctions a where rownum<50000;

Execution Plan
----------------------------------------------------------
Plan hash value: 721966694

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| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 49999 | 4833K| 163 (2)| 00:00:02 |
|* 1 | COUNT STOPKEY | | | | | |
| 2 | TABLE ACCESS FULL| EW_AUCTIONS | 49999 | 4833K| 163 (2)| 00:00:02 |
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SQL> select * from cnmir.ew_auctions where issue_id<80;

Execution Plan
----------------------------------------------------------
Plan hash value: 518049957

---------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 99 | 1379 (2)| 00:00:17 |
|* 1 | TABLE ACCESS FULL| EW_AUCTIONS | 1 | 99 | 1379 (2)| 00:00:17 |
---------------------------------------------------------------------------------

可见COUNT STOPKEY 机制是只针对rownum而设定的!