PLSQL_统计信息系列08_统计信息生成和还原

2015-02-01 Created By BaoXinjian

一、摘要

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统计信息在重新生成后，发现并不能改善程序的性能，甚至更差的时候

Oracle提供了dbms_stat包，对统计信息进行还原

1. 还原步骤如下

Step1. Regather the stats of the tables involved in the query. 重新产生统计信息;

Step2. Check the excution plan of the SQL with explain plan. 统计信息更新后解析计划效果;

Step3. If the plan changes back, ask to kill the current running job and re-run it. 如果效果可以，则杀掉该进程，重启程序;

Step4. If regather doesn't work, try to restore the stats of the table whose stats are gathered recently. 如效果不行，则对计息计划进行还原;

 

2. 获取解析计划的脚本

set pagesize 0

set linesize 150

set serveroutput on size 10000 col plan_table_output format a125 undefine hash_value set verify off feedback off

var hash_value varchar2(20) begin :hash_value := '&hash_value'; end; /

insert into plan_table (statement_id,timestamp,operation,options,object_node,object_owner,object_name, optimizer,search_columns,id,parent_id,position,cost,cardinality,bytes,other_tag, partition_start,partition_stop,partition_id,other,distribution, cpu_cost,io_cost,temp_space,access_predicates,filter_predicates, plan_id,OBJECT_ALIAS,DEPTH,PROJECTION,TIME,QBLOCK_NAME ) select distinct hash_value,sysdate,operation,options,object_node,object_owner,object_name, optimizer,search_columns,id,parent_id,position,cost,cardinality,bytes,other_tag, partition_start,partition_stop,partition_id,other,distribution, cpu_cost,io_cost,temp_space,access_predicates,filter_predicates, :hash_value,OBJECT_ALIAS,DEPTH,PROJECTION,TIME,QBLOCK_NAME from v$sql_plan where hash_value = :hash_value / col piece noprint select distinct piece,sql_text from v$sqltext where hash_value = :hash_value order by piece / @?/rdbms/admin/utlxplp.sql set linesize 80

set verify on feedback on pagesize 1000

Get Explain Plan Scripts

 

二、案例

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Step1. 获取Session对应的sql_id

  SELECT b.begin_interval_time, a.sql_id, a.plan_hash_value FROM dba_hist_sqlstat a, dba_hist_snapshot b WHERE   a.snap_id = b.snap_id AND   a.SQL_ID = '<SQL_ID>'

ORDER BY   1;

 

Step2. 获取解析计划

SQL> @getplan Enter value for hash_value: 684487124

ALTER DATABASE OPEN



----------------------------------------------------------------------------------------------------

| Id  | Operation                    | Name                | Rows  | Bytes | Cost (%CPU)| Time     |

----------------------------------------------------------------------------------------------------

|   0 | UPDATE STATEMENT             |                     |       |       |     1 (100)|          |

|   1 |  UPDATE                      | MGMT_TARGETS        |       |       |            |          |

|*  2 |   TABLE ACCESS BY INDEX ROWID| MGMT_TARGETS        |     1 |   182 |     1   (0)| 00:00:01 |

|*  3 |    INDEX UNIQUE SCAN         | MGMT_TARGETS_IDX_01 |     1 |       |     0   (0)|          |

----------------------------------------------------------------------------------------------------

 Predicate Information (identified by operation id): ---------------------------------------------------



   2 - filter(("LAST_LOAD_TIME"<:B2 OR "LAST_LOAD_TIME" IS NULL)) 3 - access("TARGET_GUID"=:B1)

 

Step3. 重新分析表，收集统计信息

BEGIN DBMS_STATS.GATHER_TABLE_STATS ( ownname => '<TABLE OWNER>', tabname => '<TABLE NAME>', degree => 8, method_opt => 'FOR ALL COLUMNS SIZE 1', cascade            => TRUE, estimate_percent => 1, GRANULARITY => 'GLOBAL AND PARTITION', no_invalidate => FALSE ); END；

 

Step4. 再次getplan获取计划，查看解析计划是否更新，更新后的解析计划是否正确

 

Step5. 若新的解析计划不正确，对统计信息进行回滚

BEGIN DBMS_STATS.RESTORE_TABLE_STATS('TABLE OWNER','TABLE NAME', <TIMESTAMP>, NO_INVALIDATE=>FALSE); END;

 

Thanks and Regards