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To:
　　请问：我先执行
exec dbms_stats.gather_table_stats('SCOTT','TEST', CASCADE => TRUE);
2 select * from test where tname='44444';
Explained.
Elapsed: 00:00:00.00
SQL> select * from table(dbms_xplan.display());
PLAN_TABLE_OUTPUT
Plan hash value: 217508114
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 581 | 13944 | 4759 (2)| 00:00:58 |
|* 1 | TABLE ACCESS FULL| TEST | 581 | 13944 | 4759 (2)| 00:00:58 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
1 - filter("TNAME"='44444')
13 rows selected.
Elapsed: 00:00:00.02
SQL> select * from test where tname='44444';
no rows selected
Elapsed: 00:00:00.34

为什么执行计划和最后的真实执行结果相差这么大。

From: ma.yi
2009.04.21 19:22