ORACLE提供了2个隐含参数_cache_stats_monitor和_optimizer_cache_stats用来控制CACHE的统计信息。
_cache_stats_monitor 用来控制是否收集缓冲区的统计信息，默认值为TRUE。
_optimizer_cache_stats用来控制CBO在确定成本的时候是否使用缓冲区的统计信息，默认值为FALSE。
这2个参数都可以在会话级别设置。看如下一个例子：
SQL> CONN TEST/TEST
Connected.
SQL> CREATE TABLE TEST AS SELECT * FROM ALL_OBJECTS;
Table created.
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(USER,'TEST');
PL/SQL procedure successfully completed.
SQL> ALTER SYSTEM FLUSH BUFFER_CACHE;
System altered.
SQL> SELECT COUNT(1) FROM TEST;
Execution Plan
----------------------------------------------------------
Plan hash value: 3467505462
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 195 (1)| 00:00:03 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| TEST | 50578 | 195 (1)| 00:00:03 |
-------------------------------------------------------------------
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
699 consistent gets
697 physical reads
0 redo size
517 bytes sent via SQL*Net to client
492 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
此时的COST值为195，基本全是物理读。
SQL> SELECT COUNT(1) FROM TEST;
Execution Plan
----------------------------------------------------------
Plan hash value: 3467505462
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 195 (1)| 00:00:03 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| TEST | 50578 | 195 (1)| 00:00:03 |
-------------------------------------------------------------------
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
699 consistent gets
0 physical reads
0 redo size
517 bytes sent via SQL*Net to client
492 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
此时全部是内存读取，但是COST值还是195。看一下设置_OPTIMIZER_CACHE_STATS=TRUE的情况：
SQL> ALTER SESSION SET "_OPTIMIZER_CACHE_STATS"=TRUE;
Session altered.
SQL> SELECT COUNT(1) FROM TEST;
Execution Plan
----------------------------------------------------------
Plan hash value: 3467505462
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------