清空Shared pool:
alter system flush shared_pool;
但为了整个内存的清空，只好将整个Oracle RAC环境的实例和数据库都关机，再重新启动。
可以采用清空buffer cache的方式来做：
alter system flush buffer_cache;
分析query plan,
explain plan for select * fromHZCZRK_JBXXB,HZCZRK_ZPXXB WHERE HZCZRK_JBXXB.RYID = HZCZRK_ZPXXB.RYID;
select plan_table_output fromtable(dbms_xplan.display());
得到结果：
| Id | Operation |Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------------
| 0| SELECT STATEMENT | | 2898K| 572M| | 240K (1)| 00:48:06 |
| 1| MERGE JOIN | | 2898K| 572M| | 240K (1)| 00:48:06 |
| 2| TABLE ACCESS BY INDEX ROWID|HZCZRK_ZPXXB | 2898K| 146M| | 120K (1)| 00:24:07 |
| 3| INDEX FULL SCAN | INDEX_HZCZRK_ZPXXB | 2898K| | | 6771 (1)| 00:01:22 |
|* 4| SORT JOIN | | 2898K| 425M| 2156M| 119K (1)| 00:24:00 |
| 5| TABLE ACCESS FULL | HZCZRK_JBXXB | 2898K| 425M| | 21194 (3)| 00:04:15 |
-----------------------------------------------------------------------------------------------------------
Predicate Information (identified byoperation id):
---------------------------------------------------
4-access("HZCZRK_JBXXB"."RYID"="HZCZRK_ZPXXB"."RYID")
filter("HZCZRK_JBXXB"."RYID"="HZCZRK_ZPXXB"."RYID")
使用set autotrace traceonly后可以查看执行计划、统计信息，如query:
SELECT COUNT(*) FROM HZCZRK_JBXXB;,执行计划、统计信息如下：
------------------------------------------------------------------------------
| Id | Operation |Name | Rows | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0| SELECT STATEMENT | | 1 | 9643 (2)| 00:01:56 |
| 1| SORT AGGREGATE | | 1 | | |
| 2| INDEX FAST FULL SCAN| SYS_C0010867| 17M| 9643 (2)| 00:01:56 |
------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
37509 consistent gets
37485 physical reads
0 redo size
537 bytes sent via SQL*Net toclient
524 bytes received via SQL*Netfrom client
2 SQL*Net roundtrips to/fromclient
0 sorts (memory)
0 sorts (disk)
1 rows processed
consistent gets代表内存消耗，physicalreads代表磁盘I/O消耗，单位都是数据块(DB_BLOCK_SIZE)