ORACLE DML执行计划频繁变更导致业务响应极慢问题的处理

最近，oracle rac维护中遇到一件很奇怪的事情，就是业务偶尔会出现极其缓慢的情况，查看服务器负载、数据库负载却很低，数据库和主机日志也没有明显报错，数据库内部也没有拥堵会话的情况。本文特此记录，以备后查！

背景： 从12月份开始，应用开始反馈，没次做新品发布，下发单据就会特别慢，后来为了临时解决问题，将业务指向到单节点，发现业务下发速度恢复正常。后来，过了一周左右的时间，单据下发速度再次很慢。但是， 查看服务器负载、数据库负载却很低，数据库和主机日志也没有明显报错，数据库内部也没有拥堵会话的情况。为了尽快恢复业务，重启了数据库服务，

业务暂时恢复正常。然而，三四天之后，也就是今天，又出现下发单据缓慢的情况。

问题分析：

1、基于以上背景，数据库和主机没有高负载，尤其是数据库内部没有会话拥堵的情况，我能想到的是SQL执行计划可能出现了变化。

2、查看业务承载节点，在执行的SQL语句

3、查看sql-5216pwt38ckhp的执行计划

--优秀的执行计划，一次单据下发3-8点能执行近30次

--变化后的低效执行计划，一次单据下发3-10点，执行只有1完多次

4、通过步骤3中变化前后的执行计划对比，我们发现

--变化前的执行计划走的主键唯一索引扫描，变化后的执行计划走的普通索引的范围扫描

--变化前的执行计划buffer get很低为11，且没有磁盘读，变化后的执行计划buffer很高为28万，出现了磁盘物理读

--变化前的执行计划唯一索引扫描后回表扫描一行数据，变化后的执行计划对索引全分区扫描后回表扫描一行数据

--变化前的执行计划单次单据下发3-8点能执行近30完成，变化后的执行计划单次单据下发3-10点才执行1万多次

问题处理：

基于以上问题分析，SQL-5216pwt38ckhp语句是update语句，且使用了绑定变量，因此，在不改变sql的情况下，我们

选择固定sql的执行计划。方法是使用脚本coe_load_sql_profile.sql进行执行计划固定，在执行计划固定的时候需要注意，

不能以sys登陆数据库执行固话执行计划，需要使用业务用户执行，固话过程中，需要输入两次sql_id，然后选择优异执行计划的plan hash value即可。

附上执行计划固定脚本coe_load_sql_profile.sql：

SPO coe_load_sql_profile.log;SET DEF ON TERM OFF ECHO ON FEED OFF VER OFF HEA ON LIN 2000 PAGES 100 LONG 8000000 LONGC 800000 TRIMS ON TI OFF TIMI OFF SERVEROUT ON SIZE 1000000 NUM 20 SQLP SQL>;SET SERVEROUT ON SIZE UNL;REMREM $Header: 215187.1 coe_load_sql_profile.sql 11.4.5.5 2013/03/01 carlos.sierra $REMREM Copyright (c) 2000-2013, Oracle Corporation. All rights reserved.REMREM AUTHORREM   carlos.sierra@oracle.comREMREM SCRIPTREM   coe_load_sql_profile.sqlREMREM DESCRIPTIONREM   This script loads a plan from a modified SQL into a Custom SQLREM   Profile for the original SQL.REM   If a good performing plan only reproduces with CBO HintsREM   then you can load the plan of the modified version of theREM   SQL into a Custom SQL Profile for the orignal SQL.REM   In other words, the original SQL can use the plan that wasREM   generated out of the SQL with hints.REMREM PRE-REQUISITESREM   1. Have in cache or AWR the text for the original SQL.REM   2. Have in cache or AWR the plan for the modified SQLREM      (usually with hints).REMREM PARAMETERSREM   1. ORIGINAL_SQL_ID (required)REM   2. MODIFIED_SQL_ID (required)REM   3. PLAN_HASH_VALUE (required)REMREM EXECUTIONREM   1. Connect into SQL*Plus as user with access to data dictionaryREM      and privileges to create SQL Profiles. Do not use SYS.REM   2. Execute script coe_load_sql_profile.sql passing first twoREM      parameters inline or until requested by script.REM   3. Provide plan hash value of the modified SQL when asked.REM   4. Use a DBA user but not SYS. Do not connect as SYS as the stagingREM      table cannot be created in SYS schema and you will receive an error:REM      ORA-19381: cannot create staging table in SYS schemaREMREM EXAMPLEREM   # sqlplus systemREM   SQL> START coe_load_sql_profile.sql gnjy0mn4y9pbm b8f3mbkd8bkghREM   SQL> START coe_load_sql_profile.sql;REMREM NOTESREM   1. This script works on 10g or higher.REM   2. For a similar script for 11g use coe_load_sql_baseline.sql,REM      which uses SQL Plan Baselines instead of Custom SQL Profiles.REM   3. For possible errors see coe_load_sql_profile.logREM   4. If you get "ORA-06532: Subscript outside of limit, ORA-06512: at line 1"REM      Then you may consider this change (only in a test and disposable system):REM      create or replace TYPE sys.sqlprof_attr AS VARRAY(5000) of VARCHAR2(500);REMSET TERM ON ECHO OFF;PROPRO Parameter 1:PRO ORIGINAL_SQL_ID (required)PRODEF original_sql_id = '&1';PROPRO Parameter 2:PRO MODIFIED_SQL_ID (required)PRODEF modified_sql_id = '&2';PROWITHp AS (SELECT plan_hash_value  FROM gv$sql_plan WHERE sql_id = TRIM('&&modified_sql_id.')   AND other_xml IS NOT NULL UNIONSELECT plan_hash_value  FROM dba_hist_sql_plan WHERE sql_id = TRIM('&&modified_sql_id.')   AND other_xml IS NOT NULL ),m AS (SELECT plan_hash_value,       SUM(elapsed_time)/SUM(executions) avg_et_secs  FROM gv$sql WHERE sql_id = TRIM('&&modified_sql_id.')   AND executions > 0 GROUP BY       plan_hash_value ),a AS (SELECT plan_hash_value,       SUM(elapsed_time_total)/SUM(executions_total) avg_et_secs  FROM dba_hist_sqlstat WHERE sql_id = TRIM('&&modified_sql_id.')   AND executions_total > 0 GROUP BY       plan_hash_value )SELECT p.plan_hash_value,       ROUND(NVL(m.avg_et_secs, a.avg_et_secs)/1e6, 3) avg_et_secs  FROM p, m, a WHERE p.plan_hash_value = m.plan_hash_value(+)   AND p.plan_hash_value = a.plan_hash_value(+) ORDER BY       avg_et_secs NULLS LAST;PROPRO Parameter 3:PRO PLAN_HASH_VALUE (required)PRODEF plan_hash_value = '&3';PROPRO Values passed to coe_load_sql_profile:PRO ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~PRO ORIGINAL_SQL_ID: "&&original_sql_id."PRO MODIFIED_SQL_ID: "&&modified_sql_id."PRO PLAN_HASH_VALUE: "&&plan_hash_value."PROWHENEVER SQLERROR EXIT SQL.SQLCODE;SET TERM OFF ECHO ON;-- trim parametersCOL original_sql_id NEW_V original_sql_id FOR A30;COL modified_sql_id NEW_V modified_sql_id FOR A30;COL plan_hash_value NEW_V plan_hash_value FOR A30;SELECT TRIM('&&original_sql_id.') original_sql_id, TRIM('&&modified_sql_id.') modified_sql_id, TRIM('&&plan_hash_value.') plan_hash_value FROM DUAL;-- open log fileSPO coe_load_sql_profile_&&original_sql_id..log;GET coe_load_sql_profile.log;.-- get userCOL connected_user NEW_V connected_user FOR A30;SELECT USER connected_user FROM DUAL;VAR sql_text CLOB;VAR other_xml CLOB;VAR signature NUMBER;VAR name VARCHAR2(30);EXEC :sql_text := NULL;EXEC :other_xml := NULL;EXEC :signature := NULL;EXEC :name := NULL;-- get sql_text from memoryDECLARE  l_sql_text VARCHAR2(32767);BEGIN -- 10g see bug 5017909  FOR i IN (SELECT DISTINCT piece, sql_text              FROM gv$sqltext_with_newlines             WHERE sql_id = TRIM('&&original_sql_id.')             ORDER BY 1, 2)  LOOP    IF :sql_text IS NULL THEN      DBMS_LOB.CREATETEMPORARY(:sql_text, TRUE);      DBMS_LOB.OPEN(:sql_text, DBMS_LOB.LOB_READWRITE);    END IF;    l_sql_text := REPLACE(i.sql_text, CHR(00), ' ');    DBMS_LOB.WRITEAPPEND(:sql_text, LENGTH(l_sql_text), l_sql_text);  END LOOP;  IF :sql_text IS NOT NULL THEN    DBMS_LOB.CLOSE(:sql_text);  END IF;EXCEPTION  WHEN OTHERS THEN    DBMS_OUTPUT.PUT_LINE('getting original sql_text from memory: '||SQLERRM);    :sql_text := NULL;END;/-- get sql_text from awrBEGIN  IF :sql_text IS NULL OR NVL(DBMS_LOB.GETLENGTH(:sql_text), 0) = 0 THEN    SELECT REPLACE(sql_text, CHR(00), ' ')      INTO :sql_text      FROM dba_hist_sqltext     WHERE sql_id = TRIM('&&original_sql_id.')       AND sql_text IS NOT NULL       AND ROWNUM = 1;  END IF;EXCEPTION  WHEN OTHERS THEN    DBMS_OUTPUT.PUT_LINE('getting original sql_text from awr: '||SQLERRM);    :sql_text := NULL;END;/-- sql_text as foundSELECT :sql_text FROM DUAL;-- check is sql_text for original sql is availableSET TERM ON;BEGIN  IF :sql_text IS NULL THEN    RAISE_APPLICATION_ERROR(-20100, 'SQL_TEXT for original SQL_ID &&original_sql_id. was not found in memory (gv$sqltext_with_newlines) or AWR (dba_hist_sqltext).');  END IF;END;/SET TERM OFF;-- get other_xml from memoryBEGIN  FOR i IN (SELECT other_xml              FROM gv$sql_plan             WHERE sql_id = TRIM('&&modified_sql_id.')               AND plan_hash_value = TO_NUMBER(TRIM('&&plan_hash_value.'))               AND other_xml IS NOT NULL             ORDER BY                   child_number, id)  LOOP    :other_xml := i.other_xml;    EXIT; -- 1st  END LOOP;EXCEPTION  WHEN OTHERS THEN    DBMS_OUTPUT.PUT_LINE('getting modified other_xml from memory: '||SQLERRM);    :other_xml := NULL;END;/-- get other_xml from awrBEGIN  IF :other_xml IS NULL OR NVL(DBMS_LOB.GETLENGTH(:other_xml), 0) = 0 THEN    FOR i IN (SELECT other_xml                FROM dba_hist_sql_plan               WHERE sql_id = TRIM('&&modified_sql_id.')                 AND plan_hash_value = TO_NUMBER(TRIM('&&plan_hash_value.'))                 AND other_xml IS NOT NULL               ORDER BY                     id)    LOOP      :other_xml := i.other_xml;      EXIT; -- 1st    END LOOP;  END IF;EXCEPTION  WHEN OTHERS THEN    DBMS_OUTPUT.PUT_LINE('getting modified other_xml from awr: '||SQLERRM);    :other_xml := NULL;END;/-- other_xml as foundSELECT :other_xml FROM DUAL;-- validate other_xmlSET TERM ON;BEGIN  IF :other_xml IS NULL THEN    RAISE_APPLICATION_ERROR(-20101, 'PLAN for modified SQL_ID &&modified_sql_id. and PHV &&plan_hash_value. was not found in memory (gv$sql_plan) or AWR (dba_hist_sql_plan).');  END IF;END;/SET ECHO OFF;DECLARE  h SYS.SQLPROF_ATTR := SYS.SQLPROF_ATTR ();  idx INTEGER := 0;  l_pos NUMBER;  l_hint VARCHAR2(32767);  description VARCHAR2(500);  PROCEDURE add_hint (p_hint IN VARCHAR2)  IS  BEGIN    idx := idx + 1;    DBMS_OUTPUT.PUT_LINE(LPAD(idx, 4, '0')||' '||p_hint);    h.EXTEND;    h(idx) := p_hint;  END add_hint;BEGIN  add_hint('BEGIN_OUTLINE_DATA');  FOR i IN (SELECT /*+ opt_param('parallel_execution_enabled', 'false') */                   SUBSTR(EXTRACTVALUE(VALUE(d), '/hint'), 1, 4000) hint              FROM TABLE(XMLSEQUENCE(EXTRACT(XMLTYPE(:other_xml), '/*/outline_data/hint'))) d)  LOOP    l_hint := i.hint;    WHILE NVL(LENGTH(l_hint), 0) > 0    LOOP      IF LENGTH(l_hint) <= 500 THEN        add_hint(l_hint);        l_hint := NULL;      ELSE        l_pos := INSTR(SUBSTR(l_hint, 1, 500), ' ', -1);        add_hint(SUBSTR(l_hint, 1, l_pos));        l_hint := '   '||SUBSTR(l_hint, l_pos);      END IF;    END LOOP;  END LOOP;  add_hint('END_OUTLINE_DATA');  :signature := DBMS_SQLTUNE.SQLTEXT_TO_SIGNATURE(:sql_text);  :name := UPPER(TRIM('&&original_sql_id.'))||'_'||TRIM('&&plan_hash_value.');  description := UPPER('original:'||TRIM('&&original_sql_id.')||' modified:'||TRIM('&&modified_sql_id.')||' phv:'||TRIM('&&plan_hash_value.')||' signature:'||:signature||' created by coe_load_sql_profile.sql');  -- create custom sql profile for original sql using plan from modified sql  DBMS_SQLTUNE.IMPORT_SQL_PROFILE (    sql_text    => :sql_text, -- original sql    profile     => h, -- plan from modified sql    name        => :name,    description => description,    category    => 'DEFAULT',    validate    => TRUE,    replace     => TRUE,    force_match => FALSE /* TRUE:FORCE (match even when different literals in SQL). FALSE:EXACT (similar to CURSOR_SHARING) */ );  -- drop sql profile staging table for original sql (if one exists)  BEGIN    DBMS_OUTPUT.PUT_LINE('dropping staging table "STGTAB_SQLPROF_'||UPPER(TRIM('&&original_sql_id.'))||'"');    EXECUTE IMMEDIATE 'DROP TABLE STGTAB_SQLPROF_'||UPPER(TRIM('&&original_sql_id.'));  EXCEPTION    WHEN OTHERS THEN      DBMS_OUTPUT.PUT_LINE('staging table "STGTAB_SQLPROF_'||UPPER(TRIM('&&original_sql_id.'))||'" did not exist');  END;  -- create sql profile staging table for original sql  DBMS_OUTPUT.PUT_LINE('creating staging table "STGTAB_SQLPROF_'||UPPER(TRIM('&&original_sql_id.'))||'"');  DBMS_SQLTUNE.CREATE_STGTAB_SQLPROF  (    table_name  => 'STGTAB_SQLPROF_'||UPPER(TRIM('&&original_sql_id.')),    schema_name => '&&connected_user.' );  -- packs new sql profile for original sql  DBMS_OUTPUT.PUT_LINE('packaging new sql profile into staging table "STGTAB_SQLPROF_'||UPPER(TRIM('&&original_sql_id.'))||'"');  DBMS_SQLTUNE.PACK_STGTAB_SQLPROF (     profile_name         => :name,     staging_table_name   => 'STGTAB_SQLPROF_'||UPPER(TRIM('&&original_sql_id.')),     staging_schema_owner => '&&connected_user.' );END;/-- profile_nameCOL profile_name NEW_V profile_name FOR A30;SELECT :name profile_name FROM DUAL;-- display details of new sql_profileSET ECHO ON;REMREM SQL ProfileREM ~~~~~~~~~~~REMSELECT signature, name, category, type, status  FROM dba_sql_profiles WHERE name = :name;SELECT description  FROM dba_sql_profiles WHERE name = :name;SET ECHO OFF;PROPRO ****************************************************************************PRO * Enter &&connected_user. password to export staging table STGTAB_SQLPROF_&&original_sql_id.PRO ****************************************************************************HOS exp &&connected_user. tables=&&connected_user..STGTAB_SQLPROF_&&original_sql_id. file=STGTAB_SQLPROF_&&original_sql_id..dmp statistics=NONE indexes=N constraints=N grants=N triggers=NPROPRO If you need to implement this Custom SQL Profile on a similar system,PRO import and unpack using these commands:PROPRO imp &&connected_user. file=STGTAB_SQLPROF_&&original_sql_id..dmp tables=STGTAB_SQLPROF_&&original_sql_id. ignore=YPROPRO BEGINPRO   DBMS_SQLTUNE.UNPACK_STGTAB_SQLPROF (PRO     profile_name => '&&profile_name.',PRO     replace => TRUE,PRO     staging_table_name => 'STGTAB_SQLPROF_&&original_sql_id.',PRO     staging_schema_owner => '&&connected_user.' );;PRO END;;PRO /PROSPO OFF;HOS zip -m coe_load_sql_profile_&&original_sql_id. coe_load_sql_profile_&&original_sql_id..log STGTAB_SQLPROF_&&original_sql_id..dmp coe_load_sql_profile.logHOS zip -d coe_load_sql_profile_&&original_sql_id. coe_load_sql_profile.logWHENEVER SQLERROR CONTINUE;SET DEF ON TERM ON ECHO OFF FEED 6 VER ON HEA ON LIN 80 PAGES 14 LONG 80 LONGC 80 TRIMS OFF TI OFF TIMI OFF SERVEROUT OFF NUM 10 SQLP SQL>;SET SERVEROUT OFF;UNDEFINE 1 2 3 original_sql_id modified_sql_id plan_hash_value profile_nameCL COLPROPRO coe_load_sql_profile completed.

问题处理结果：