业务人员告诉我某系统磁盘IO持续高达300MB/s,系统平台为AIX,遂 topas 查看果然如此。
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
[email protected]>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 / [email protected]$1 4 - SEL$1 / [email protected]$1 5 - SEL$1 / [email protected]$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:
[email protected]>select num_rows,last_analyzed from dba_tables where table_name='zbdba1'; NUM_ROWS LAST_ANALYZE ---------- ------------ 659764063 31-JUL-15 [email protected]>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: [email protected]>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 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');
重新利用合适的绑定变量跑出正确的执行计划即可。
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。
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