UNDO是用于实现并发控制以及构建一致性读，也就是在数据变更之前产生前镜像，以保证用户能够回滚或撤销对数据库所作的修改。是Oracle数据库完整性的重要组成部分。因此合理的设计及配置undo以及使用undo都将对数据库有较大的影响。通常情况下，对于大规模数据的删除，更新操作，我们建议使用分批删除分次提交以减少对undo的占用和冲击。那么对于undo的大小到底应该设置多大？是启用自动扩展还是关闭自动扩展？这个问题仁者见仁，智者见智，见下文。
1、当前数据库环境及undo配置信息
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[sql]
sys@SYTST> select * from v$version where rownum<2;
BANNER
----------------------------------------------------------------
Oracle Database 10g Release 10.2.0.3.0 - 64bit Production
sys@SYTST> show parameter undo
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
undo_management string AUTO
undo_retention integer 900
undo_tablespace string UNDOTBS
sys@SYTST> select tablespace_name,file_name,AUTOEXTENSIBLE,bytes/1024/1024 size_mb
2 from dba_data_files where tablespace_name like '%UNDO%';
TABLESPACE_NAME FILE_NAME AUT SIZE_MB
------------------------------ ------------------------------------------------------- --- ----------
UNDOTBS /u02/database/SYTST/undo/undoSYTST1.dbf NO 20
--创建演示表t
sys@SYTST> CREATE TABLE t
2 AS
3 SELECT rownum AS id,
4 round(5678+dbms_random.normal*1234) AS n1,
5 mod(255+trunc(dbms_random.normal*1000),255) AS n2,
6 dbms_random.string('p',255) AS pad
7 FROM dual
8 CONNECT BY level <= 100000
9 ORDER BY dbms_random.value;
Table created.
--直接用脚本得到undo的信息及建议值
sys@SYTST> @chk_advs_undo
Session altered.
- Undo Analysis started at : 24/10/2013 14:39:58 -
--------------------------------------------------
NOTE:The following analysis is based upon the database workload during the period -
Begin Time : 17/10/2013 14:39:58
End Time : 24/10/2013 14:39:58
Current Undo Configuration
--------------------------
Current undo tablespace : UNDOTBS
Current undo tablespace size (datafile size now) : 20M
Current undo tablespace size (consider autoextend) : 20M
AUTOEXTEND for undo tablespace is : OFF
Current undo retention : 900
UNDO GUARANTEE is set to : FALSE
Undo Advisor Summary
---------------------------
Finding 1:The undo tablespace is OK. --->当前的undo配置合理
Undo Space Recommendation
-------------------------
Allocated undo space is sufficient for the current workload.
Retention Recommendation
------------------------
The best possible retention with current configuration is : 5996 Seconds
The longest running query ran for : 52 Seconds
The undo retention required to avoid errors is : 52 Seconds
PL/SQL procedure successfully completed.
2、模拟undo超出并获得建议值
[sql]
--先查看当前已产生的undo
sys@SYTST> @mystat "undo change"
sys@SYTST> set echo off
NAME VALUE
---------------------------------------------------------------- ----------
undo change vector size 363568
--我们来更新之前创建表t上的pad列
sys@SYTST> update t set pad=dbms_random.string('l',255);
update t set pad=dbms_random.string('l',255)
*
ERROR at line 1:
ORA-30036: unable to extend segment by 8 in undo tablespace 'UNDOTBS' --->得到了错误提示，无法扩展undo，因为当前环境undo未启用自动扩展
--Author : Leshami
--Blog : https://blog.csdn.net/leshami
--看看update语句到底产生了多少undo
sys@SYTST> @mystat2
sys@SYTST> set echo off
NAME V DIFF
---------------------------------------------------------------- ---------- ----------------
undo change vector size 18817316 18,453,748
--由下可知，上面的update语句已经产生了17M以上的undo,而当前的undo的大小为20M，显然不够，所以我们收到了ORA-30036
sys@SYTST> select 18453748/1024/1024 from dual;
18453748/1024/1024
------------------
17.5988655
--再次获得undo建议值
sys@SYTST> @chk_advs_undo
Session altered.
- Undo Analysis started at : 24/10/2013 14:49:07 -
--------------------------------------------------
NOTE:The following analysis is based upon the database workload during the period -
Begin Time : 17/10/2013 14:49:07
End Time : 24/10/2013 14:49:07
Current Undo Configuration
--------------------------
Current undo tablespace : UNDOTBS
Current undo tablespace size (datafile size now) : 20M
Current undo tablespace size (consider autoextend) : 20M
AUTOEXTEND for undo tablespace is : OFF
Current undo retention : 900
UNDO GUARANTEE is set to : FALSE
Undo Advisor Summary
---------------------------
Finding 1:Undo Tablespace is under pressure. Recommendation 1:Size undo tablespace to 48 MB
Undo Space Recommendation
-------------------------
Minimum Recommendation : Size undo tablespace to 48 MB
Rationale : Increase undo tablespace size so that long running queries will not fail
Recommended Undo Tablespace Size : 48M --->这里给出了undo的建议值
Retention Recommendation
------------------------ --->下面是一些和undo相关的建议，如可能的保留时间等等
The best possible retention with current configuration is : 306 Seconds
The longest running query ran for : 52 Seconds
The undo retention required to avoid errors is : 52 Seconds
PL/SQL procedure successfully completed.
3、获得undo信息及建议值脚本
[sql]
--脚本来自Oracle. 参考Metalink: Doc ID 1579035.1
--以下脚本适用于Oracle 10g以上版本
SET SERVEROUTPUT ON
SET LINES 600
ALTER SESSION SET NLS_DATE_FORMAT = 'DD/MM/YYYY HH24:MI:SS';
DECLARE
v_analyse_start_time DATE := SYSDATE - 7;
v_analyse_end_time DATE := SYSDATE;
v_cur_dt DATE;
v_undo_info_ret BOOLEAN;
v_cur_undo_mb NUMBER;
v_undo_tbs_name VARCHAR2(100);
v_undo_tbs_size NUMBER;
v_undo_autoext BOOLEAN;
v_undo_retention NUMBER(6);
v_undo_guarantee BOOLEAN;
v_instance_number NUMBER;
v_undo_advisor_advice VARCHAR2(100);
v_undo_health_ret NUMBER;
v_problem VARCHAR2(1000);
v_recommendation VARCHAR2(1000);
v_rationale VARCHAR2(1000);
v_retention NUMBER;
v_utbsize NUMBER;
v_best_retention NUMBER;
v_longest_query NUMBER;
v_required_retention NUMBER;
BEGIN
select sysdate into v_cur_dt from dual;
DBMS_OUTPUT.PUT_LINE(CHR(9));
DBMS_OUTPUT.PUT_LINE('- Undo Analysis started at : ' || v_cur_dt || ' -');
DBMS_OUTPUT.PUT_LINE('--------------------------------------------------');
v_undo_info_ret := DBMS_UNDO_ADV.UNDO_INFO(v_undo_tbs_name, v_undo_tbs_size, v_undo_autoext, v_undo_retention, v_undo_guarantee);
select sum(bytes)/1024/1024 into v_cur_undo_mb from dba_data_files where tablespace_name = v_undo_tbs_name;
DBMS_OUTPUT.PUT_LINE('NOTE:The following analysis is based upon the database workload during the period -');
DBMS_OUTPUT.PUT_LINE('Begin Time : ' || v_analyse_start_time);
DBMS_OUTPUT.PUT_LINE('End Time : ' || v_analyse_end_time);
DBMS_OUTPUT.PUT_LINE(CHR(9));
DBMS_OUTPUT.PUT_LINE('Current Undo Configuration');
DBMS_OUTPUT.PUT_LINE('--------------------------');
DBMS_OUTPUT.PUT_LINE(RPAD('Current undo tablespace',55) || ' : ' || v_undo_tbs_name);
DBMS_OUTPUT.PUT_LINE(RPAD('Current undo tablespace size (datafile size now) ',55) || ' : ' || v_cur_undo_mb || 'M');
DBMS_OUTPUT.PUT_LINE(RPAD('Current undo tablespace size (consider autoextend) ',55) || ' : ' || v_undo_tbs_size || 'M');
IF V_UNDO_AUTOEXT THEN
DBMS_OUTPUT.PUT_LINE(RPAD('AUTOEXTEND for undo tablespace is',55) || ' : ON');
ELSE
DBMS_OUTPUT.PUT_LINE(RPAD('AUTOEXTEND for undo tablespace is',55) || ' : OFF');
END IF;
DBMS_OUTPUT.PUT_LINE(RPAD('Current undo retention',55) || ' : ' || v_undo_retention);
IF v_undo_guarantee THEN
DBMS_OUTPUT.PUT_LINE(RPAD('UNDO GUARANTEE is set to',55) || ' : TRUE');
ELSE
dbms_output.put_line(RPAD('UNDO GUARANTEE is set to',55) || ' : FALSE');
END IF;
DBMS_OUTPUT.PUT_LINE(CHR(9));
SELECT instance_number INTO v_instance_number FROM V$INSTANCE;
DBMS_OUTPUT.PUT_LINE('Undo Advisor Summary');
DBMS_OUTPUT.PUT_LINE('---------------------------');
v_undo_advisor_advice := dbms_undo_adv.undo_advisor(v_analyse_start_time, v_analyse_end_time, v_instance_number);
DBMS_OUTPUT.PUT_LINE(v_undo_advisor_advice);
DBMS_OUTPUT.PUT_LINE(CHR(9));
DBMS_OUTPUT.PUT_LINE('Undo Space Recommendation');
DBMS_OUTPUT.PUT_LINE('-------------------------');
v_undo_health_ret := dbms_undo_adv.undo_health(v_analyse_start_time, v_analyse_end_time, v_problem, v_recommendation, v_rationale, v_retention, v_utbsize);
IF v_undo_health_ret > 0 THEN
DBMS_OUTPUT.PUT_LINE('Minimum Recommendation : ' || v_recommendation);
DBMS_OUTPUT.PUT_LINE('Rationale : ' || v_rationale);
DBMS_OUTPUT.PUT_LINE('Recommended Undo Tablespace Size : ' || v_utbsize || 'M');
ELSE
DBMS_OUTPUT.PUT_LINE('Allocated undo space is sufficient for the current workload.');
END IF;
SELECT dbms_undo_adv.best_possible_retention(v_analyse_start_time, v_analyse_end_time) into v_best_retention FROM dual;
SELECT dbms_undo_adv.longest_query(v_analyse_start_time, v_analyse_end_time) into v_longest_query FROM dual;
SELECT dbms_undo_adv.required_retention(v_analyse_start_time, v_analyse_end_time) into v_required_retention FROM dual;
DBMS_OUTPUT.PUT_LINE(CHR(9));
DBMS_OUTPUT.PUT_LINE('Retention Recommendation');
DBMS_OUTPUT.PUT_LINE('------------------------');
DBMS_OUTPUT.PUT_LINE(RPAD('The best possible retention with current configuration is ',60) || ' : ' || v_best_retention || ' Seconds');
DBMS_OUTPUT.PUT_LINE(RPAD('The longest running query ran for ',60) || ' : ' || v_longest_query || ' Seconds');
DBMS_OUTPUT.PUT_LINE(RPAD('The undo retention required to avoid errors is ',60) || ' : ' || v_required_retention || ' Seconds');
END;
/
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4、后记
a、undo究竟要多大，推荐的做法是根据你的业务高峰期得到一个峰值，然后适当高于峰值来启用自动扩展。
b、undo保留时间与所需的空间成正比，保留时间越长，空间开销越大，不过出现ORA-01555的几率也就越低。
c、在未启用自动扩展的情形下，对于新的事务，UNDO_RETENTION会回收空闲空间，并重新使用，如果启用自动扩展，则该功能被忽略，而是先扩展。
d、小的undo表空间(数据文件)通常情况下对于数据库及操作系统影响更小，具有更好的性能以及减少备份所需的开销等。
转自：https://www.2cto.com/database/201310/252699.html