用下面脚本到Oracle数据库中看了一下：
SELECT Disk_Reads DiskReads, Executions, SQL_ID, SQL_Text SQLText,
SQL_FullText SQLFullText
FROM
(
SELECT Disk_Reads, Executions, SQL_ID, LTRIM(SQL_Text) SQL_Text,
SQL_FullText, Operation, Options,
Row_Number() OVER
(Partition By sql_text ORDER BY Disk_Reads * Executions DESC)
KeepHighSQL
FROM
(
SELECT Avg(Disk_Reads) OVER (Partition By sql_text) Disk_Reads,
Max(Executions) OVER (Partition By sql_text) Executions,
t.SQL_ID, sql_text, sql_fulltext, p.operation,p.options
FROM v$sql t, v$sql_plan p
WHERE t.hash_value=p.hash_value AND p.operation='TABLE ACCESS'
AND p.options='FULL' AND p.object_owner NOT IN ('SYS','SYSTEM')
AND t.Executions > 1
)
ORDER BY DISK_READS * EXECUTIONS DESC
)
WHERE KeepHighSQL = 1
AND rownum <=5;
这里当时没有保存记录，总之跟后来AWR收集的是一样的sql，如下所示。
可以看到第一个sql的物理读非常高。遂看了一下执行计划
SYS@zbdba>select * from table(dbms_xplan.display_cursor('54043712',null,'advanced'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
HASH_VALUE 54043712, child number 0
------------------------------------
SELECT D.MODEL_ID, D.OBJ_ID, D.OBJ_TYPE, D.DATA_TYPE, D.DATA_DATE, D.DATA_FROM_DATE, D.DATA_TO_DATE,
D.DATA_FLAG FROM zbdba1 D, zbdba2 C WHERE D.MODEL_ID = C.MODEL_ID AND C.COLLECT_ID =
:COLLECT_ID AND D.DATA_DATE = :DATA_DATE AND D.DATA_TYPE = :DATA_TYPE AND D.VALUE_FLAG = 0
Plan hash value: 1780662521
-------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 9 (100)| | | |
| 1 | NESTED LOOPS | | 1 | 61 | 9 (0)| 00:00:01 | | |
| 2 | PARTITION RANGE SINGLE | | 1 | 51 | 8 (0)| 00:00:01 | KEY | KEY |
|* 3 | TABLE ACCESS FULL | zbdba1 | 1 | 51 | 8 (0)| 00:00:01 | KEY | KEY |
|* 4 | TABLE ACCESS BY INDEX ROWID| zbdba2 | 1 | 10 | 1 (0)| 00:00:01 | | |
|* 5 | INDEX UNIQUE SCAN | PK_zbdba2 | 1 | | 0 (0)| | | |
-------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
3 - SEL$1 / D@SEL$1
4 - SEL$1 / C@SEL$1
5 - SEL$1 / C@SEL$1
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('10.2.0.5')
ALL_ROWS
OUTLINE_LEAF(@"SEL$1")
FULL(@"SEL$1" "D"@"SEL$1")
INDEX_RS_ASC(@"SEL$1" "C"@"SEL$1" ("zbdba2"."MODEL_ID"))
LEADING(@"SEL$1" "D"@"SEL$1" "C"@"SEL$1")
USE_NL(@"SEL$1" "C"@"SEL$1")
END_OUTLINE_DATA
*/
Peeked Binds (identified by position):
--------------------------------------
1 - :COLLECT_ID (VARCHAR2(30), CSID=852): '70350'
2 - :DATA_DATE (DATE): 06/01/15 00:00:00
3 - :DATA_TYPE (VARCHAR2(30), CSID=852): '02'
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter(("D"."DATA_TYPE"=:DATA_TYPE AND "D"."DATA_DATE"=:DATA_DATE AND "D"."VALUE_FLAG"=0))
4 - filter("C"."COLLECT_ID"=TO_NUMBER(:COLLECT_ID))
5 - access("D"."MODEL_ID"="C"."MODEL_ID")
Column Projection Information (identified by operation id):
-----------------------------------------------------------
1 - "D"."MODEL_ID"[NUMBER,22], "D"."OBJ_ID"[NUMBER,22], "D"."OBJ_TYPE"[VARCHAR2,2],
"D"."DATA_TYPE"[VARCHAR2,2], "D"."DATA_DATE"[DATE,7], "D"."DATA_FROM_DATE"[DATE,7], "D"."DATA_TO_DATE"[DATE,7],
"D"."DATA_FLAG"[NUMBER,22]
2 - "D"."MODEL_ID"[NUMBER,22], "D"."OBJ_ID"[NUMBER,22], "D"."OBJ_TYPE"[VARCHAR2,2],
"D"."DATA_TYPE"[VARCHAR2,2], "D"."DATA_DATE"[DATE,7], "D"."DATA_FROM_DATE"[DATE,7], "D"."DATA_TO_DATE"[DATE,7],
"D"."DATA_FLAG"[NUMBER,22]
3 - "D"."MODEL_ID"[NUMBER,22], "D"."OBJ_ID"[NUMBER,22], "D"."OBJ_TYPE"[VARCHAR2,2],
"D"."DATA_TYPE"[VARCHAR2,2], "D"."DATA_DATE"[DATE,7], "D"."DATA_FROM_DATE"[DATE,7], "D"."DATA_TO_DATE"[DATE,7],
"D"."DATA_FLAG"[NUMBER,22]
5 - "C".ROWID[ROWID,10]
HASH_VALUE 54043712, child number 1
SELECT D.MODEL_ID, D.OBJ_ID, D.OBJ_TYPE, D.DATA_TYPE, D.DATA_DATE, D.DATA_FROM_DATE, D.DATA_TO_DATE,
D.DATA_FLAG FROM zbdba1 D, zbdba2 C WHERE D.MODEL_ID = C.MODEL_ID AND C.COLLECT_ID =
:COLLECT_ID AND D.DATA_DATE = :DATA_DATE AND D.DATA_TYPE = :DATA_TYPE AND D.VALUE_FLAG = 0
NOTE: cannot fetch plan for HASH_VALUE: 54043712, CHILD_NUMBER: 1
Please verify value of HASH_VALUE and CHILD_NUMBER;
It could also be that the plan is no longer in cursor cache (check v$sql_plan)
81 rows selected.
查看表行数：
SYS@zbdba>select num_rows,last_analyzed from dba_tables where table_name='zbdba1';
NUM_ROWS LAST_ANALYZE
---------- ------------
659764063 31-JUL-15
SYS@zbdba>select num_rows,last_analyzed from dba_tables where table_name='zbdba2';
NUM_ROWS LAST_ANALYZE
---------- ------------
76513 14-JUL-15
explan plan for 该sql查看执行计划：SYS@zbdba>select * from table(dbms_xplan.display());PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Plan hash value: 2057366878----------------------------------------------------------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |----------------------------------------------------------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 3 | 183 | 11 (0)| 00:00:01 | | || 1 | TABLE ACCESS BY GLOBAL INDEX ROWID| zbdba1 | 1 | 51 | 4 (0)| 00:00:01 | ROWID | ROWID || 2 | NESTED LOOPS | | 3 | 183 | 11 (0)| 00:00:01 | | || 3 | TABLE ACCESS BY INDEX ROWID | zbdba2 | 2 | 20 | 4 (0)| 00:00:01 | | ||* 4 | INDEX RANGE SCAN | IDX_zbdba2COLLECTFLAG | 2 | | 2 (0)| 00:00:01 | | ||* 5 | INDEX RANGE SCAN | IDX_C_COLLECT_MD_001 | 1 | | 3 (0)| 00:00:01 | | |----------------------------------------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):--------------------------------------------------- 4 - access("C"."COLLECT_ID"=TO_NUMBER(:COLLECT_ID)) 5 - access("D"."MODEL_ID"="C"."MODEL_ID" AND "D"."DATA_DATE"=:DATA_DATE AND "D"."DATA_TYPE"=:DATA_TYPE AND "D"."VALUE_FLAG"=0)19 rows selected.
发现走了索引，但是当时为什么没有走索引呢。并且在二节点走了全表扫描，一节点走的是索引范围扫描。
该oracle数据库的版本为10.2.0.5，在11g之前没有引入ACS（Adaptive Cursor Sharing），所以这里CBO在第一次进行硬解析的时候才会窥视变量的值，并且生成执行计划，之后一直使用相同的执行计划。
这里我猜想，在2节点。第一次使用绑定变量的时候，CBO认为应该使用全表扫描效率更高，所以在以后一直使用该执行计划。然而在一节点，第一次使用绑定变量的时候，CBO认为走范围扫描效率更高。所以这里导致1节点和2节点的执行计划不一样。
找到原因了，就好办了。业务人员怕数据库负载过大导致宕机，遂叫我把该sql的相关进程（发现30个进程）全部kill掉。kill完进程后磁盘IO瞬间降到50MB。
还没有完，改sql以后还会有这种选择。我们怎么去避免？
既然了解了CBO的做法，那就触发它再一次去执行一次硬解析获得正确的执行计划。有如下4种方法：
1、alter system flush shared_pool（想跪就跑这个）
2、对相关表做DDL操作
3、重新收集统计信息
4、dbms_shared_pool.purge
前面三种方案对生产系统都影响比较大，所以利用第四种方法。
SQL> select address,hash_value,executionsfrom v$sql where hash_value=54043712重新利用合适的绑定变量跑出正确的执行计划即可。
ADDRESS HASH_VALUE EXECUTIONS
---------------- ---------- ---------- -----------
0000040229F039E0 54043712 1
SQL> alter session set events '5614566 trace name context forever';
SQL> exec dbms_shared_pool.purge('0000040229F039E0,54043712','C');
如何永久保持不变呢？
加hint，强制走索引。
explain plan for SELECT /*+ index(zbdba1,IDX_C_COLLECT_MD_001)* / D.MODEL_ID,
D.OBJ_ID,
D.OBJ_TYPE,
D.DATA_TYPE,
D.DATA_DATE,
D.DATA_FROM_DATE,
D.DATA_TO_DATE,
D.DATA_FLAG
FROM EIC2.zbdba1 D, EIC2.zbdba2 C
WHERE D.MODEL_ID = C.MODEL_ID
AND C.COLLECT_ID = '70350'
AND D.DATA_DATE = to_date('06/01/15','MM/DD/YY')
AND D.DATA_TYPE = '02'
AND D.VALUE_FLAG = 0;
如果以后该索引有变化，也将会失效。
当然优化没有一劳永逸的事情，针对大小超过50GB，数据量高达6亿条的表还是要定期检查它相关sql的执行计划。