在ORACLE數(shù)據(jù)庫中，序列（SEQUENCE）是使用非常頻繁的一個數(shù)據(jù)庫對象，但是有時候會遇到序列（SEQUECNE）跳號(skip sequence numbers)的情形，那么在哪些情形下會遇到跳號呢？
事務回滾引起的跳號
不管序列有沒有CACHE、事務回滾這種情況下，都會引起序列的跳號。如下實驗所示：
SQL> create sequence my_sequence
2 start with 1
3 increment by 1
4 maxvalue 99999
5 nocache;
Sequence created.
SQL> create table test(id number(10), name varchar2(32));
Table created.
SQL> insert into test
2 select my_sequence.nextval , 'kerry' from dual;
1 row created.
SQL>
SQL> rollback;
Rollback complete.
SQL> select my_sequence.nextval from dual;
NEXTVAL
----------
3
SQL>
并發(fā)訪問序列引起的跳號
并發(fā)訪問序列引起的跳號，其實不算真正的跳號，而只是邏輯跳號，只是序列值被其它并發(fā)會話使用了。我們來構(gòu)造一起并發(fā)訪問序列引起的跳號，我們開啟兩個會話窗口，循環(huán)獲取序列的值，模擬并發(fā)出現(xiàn)的場景。
會話窗口A：
exec dbms_lock.sleep(2); --延遲2秒執(zhí)行，根據(jù)你實驗情況調(diào)整
/
begin
for i in 1 .. 2000 loop
dbms_output.put_line(my_sequence.nextval);
end loop;
end;
/
會話窗口B：
spool test.txt;
begin
waitfor delay '00:00:10';
for i in 1 .. 2000 loop
dbms_output.put_line(my_sequence.nextval);
end loop;
end;
/
spool off;
如下所示，我構(gòu)造的實驗當中，你會看到序列的跳號情況。
FLUSH SHARED_POOL會導致CACHE的序列跳號
實驗測試如下所示（序列的CACHE值必須大于0），當然正常情況下，很難遇到這種情況。
SQL> select test.my_sequence.nextval from dual;
NEXTVAL
----------
17004
SQL> alter sequence test.my_sequence cache 40;
Sequence altered.
SQL> select test.my_sequence.nextval from dual;
NEXTVAL
----------
17005
SQL> alter system flush share_pool;
alter system flush share_pool
*
ERROR at line 1:
ORA-02000: missing SHARED_POOL/BUFFER_CACHE/GLOBAL CONTEXT keyword
SQL> alter system flush shared_pool;
System altered.
SQL> select test.my_sequence.nextval from dual;
NEXTVAL
----------
17045
數(shù)據(jù)庫實例異常關(guān)閉導致跳號
如下實驗所示，當數(shù)據(jù)庫使用shutdown abort命令關(guān)閉后，重新啟動實例，序列緩存在shared pool里面沒有用過的值都沒有了。一下子從17045跳到17085
SQL> select test.my_sequence.currval from dual;
CURRVAL
----------
17045
SQL> select object_id from dba_objects where object_name=upper('my_sequence');
OBJECT_ID
----------
97760
SQL> select increment$, minvalue, maxvalue,highwater, cache
2 from seq$ where obj#=97760;
INCREMENT$ MINVALUE MAXVALUE HIGHWATER CACHE
---------- ---------- ---------- ---------- ----------
1 1 99999 17085 40
SQL> shutdown abort;
ORACLE instance shut down.
SQL> startup;
ORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instance
ORACLE instance started.
Total System Global Area 1620115456 bytes
Fixed Size 2213816 bytes
Variable Size 1258293320 bytes
Database Buffers 352321536 bytes
Redo Buffers 7286784 bytes
Database mounted.
Database opened.
SQL> select test.my_sequence.currval from dual;
select test.my_sequence.currval from dual
*
ERROR at line 1:
ORA-08002: sequence MY_SEQUENCE.CURRVAL is not yet defined in this session
SQL> select test.my_sequence.nextval from dual;
NEXTVAL
----------
17085
SQL>
另外，我們也來看看正常關(guān)閉數(shù)據(jù)庫的情況下，序列會不會出現(xiàn)跳號，我們采用10046跟蹤事件，看看正常數(shù)據(jù)庫關(guān)閉情況下，會對序列做一些啥操作
SQL> select test.my_sequence.nextval from dual;
NEXTVAL
----------
17085
SQL> alter session set events '10046 trace name context forever, level 4';
Session altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instance
ORACLE instance started.
Total System Global Area 1620115456 bytes
Fixed Size 2213816 bytes
Variable Size 1258293320 bytes
Database Buffers 352321536 bytes
Redo Buffers 7286784 bytes
Database mounted.
Database opened.
SQL> select test.my_sequence.currval from dual;
select test.my_sequence.currval from dual
*
ERROR at line 1:
ORA-08002: sequence MY_SEQUENCE.CURRVAL is not yet defined in this session
SQL> select test.my_sequence.nextval from dual;
NEXTVAL
----------
17086
SQL>
Trace文件中有更新seq$數(shù)據(jù)字典表，如果你看過我這篇文章ORACLE中seq$表更新頻繁的分析，基本上就知道其實seq$中維護的是序列的一些信息。通過跟蹤文件，我們知道在數(shù)據(jù)庫正常關(guān)閉的情況下,會觸發(fā)一個update seq$的操作,把當前的sequence.nextval的值更新到seq$.highwater中,從而使得sequence在有cache的情況下，數(shù)據(jù)庫正常關(guān)閉未出現(xiàn)nextval跳躍(currval也同樣不跳躍)；而在數(shù)據(jù)庫異常關(guān)閉之時，數(shù)據(jù)庫不能及時將sequence.nextval更新到eq$.highwater從而引起sequence cache中的值丟失，從而可能出現(xiàn)了sequence使用cache導致跳躍的情況
=====================
PARSING IN CURSOR #25 len=129 dep=1 uid=0 oct=6 lid=0 tim=1504236336294194 hv=2635489469 ad='bf780410' sqlid='4m7m0t6fjcs5x'
update seq$ set increment$=:2,minvalue=:3,maxvalue=:4,cycle#=:5,order$=:6,cache=:7,highwater=:8,audit$=:9,flags=:10 where obj#=:1
END OF STMT
PARSE #25:c=0,e=21,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=4,plh=1935744642,tim=1504236336294194
BINDS #25:
Bind#0
oacdty=02 mxl=22(02) mxlc=00 mal=00 scl=00 pre=00
oacflg=18 fl2=0001 frm=00 csi=00 siz=24 off=0
kxsbbbfp=bf45ca48 bln=24 avl=02 flg=09
value=1
Bind#1
oacdty=02 mxl=22(02) mxlc=00 mal=00 scl=00 pre=00
oacflg=18 fl2=0001 frm=00 csi=00 siz=24 off=0
kxsbbbfp=bf45ca5a bln=24 avl=02 flg=09
:/17086
Bind#3
oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00
oacflg=08 fl2=0001 frm=00 csi=00 siz=24 off=0
kxsbbbfp=2b7d80f57350 bln=24 avl=01 flg=05
value=0
Bind#4
oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00
oacflg=08 fl2=0001 frm=00 csi=00 siz=24 off=0
kxsbbbfp=2b7d80f57320 bln=24 avl=01 flg=05
value=0
Bind#5
oacdty=02 mxl=22(02) mxlc=00 mal=00 scl=00 pre=00
oacflg=18 fl2=0001 frm=00 csi=00 siz=24 off=0
kxsbbbfp=bf47b85e bln=24 avl=02 flg=09
value=40
Bind#6
oacdty=02 mxl=22(04) mxlc=00 mal=00 scl=00 pre=00
oacflg=18 fl2=0001 frm=00 csi=00 siz=24 off=0
kxsbbbfp=bf47b870 bln=24 avl=04 flg=09
value=17086
Bind#7
oacdty=01 mxl=32(32) mxlc=00 mal=00 scl=00 pre=00
oacflg=18 fl2=0001 frm=01 csi=873 siz=32 off=0
kxsbbbfp=bf47b882 bln=32 avl=32 flg=09
value="--------------------------------"
Bind#8
oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00
oacflg=08 fl2=0001 frm=00 csi=00 siz=24 off=0
kxsbbbfp=2b7d80f572f0 bln=24 avl=02 flg=05
value=8
Bind#9
oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00
oacflg=08 fl2=0001 frm=00 csi=00 siz=24 off=0
kxsbbbfp=2b7d80f57380 bln=22 avl=04 flg=05
value=97760
EXEC #9:c=999,e=709,p=0,cr=1,cu=2,mis=0,r=1,dep=1,og=4,plh=1935744642,tim=1504236336297033
CLOSE #9:c=0,e=2,dep=1,type=3,tim=1504236336297058
mealink上提到了使用dbms_shared_pool.keep將對象在鎖定在shared pool 中，永遠不釋放。這樣可以防止FLUSH SHARED POOL導致序列跳號，但是這個無法避免數(shù)據(jù)庫異常關(guān)閉或CRASH引起的跳號
SQL> select test.my_sequence.currval from dual;
CURRVAL
----------
17086
SQL> exec dbms_shared_pool.keep('test.my_sequence','q');
PL/SQL procedure successfully completed.
SQL> alter system flush shared_pool;
System altered.
SQL> select test.my_sequence.currval from dual;
CURRVAL
----------
17086
SQL> shutdown abort
ORACLE instance shut down.
SQL> startup
ORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instance
ORACLE instance started.
Total System Global Area 1620115456 bytes
Fixed Size 2213816 bytes
Variable Size 1258293320 bytes
Database Buffers 352321536 bytes
Redo Buffers 7286784 bytes
Database mounted.
Database opened.
SQL> select test.my_sequence.nextval from dual;
NEXTVAL
----------
17126
其實如果業(yè)務允許，單號出現(xiàn)跳號也無所謂的情形最好，如果碰到業(yè)務要求絕對不能出現(xiàn)單號出現(xiàn)跳號的情況，那么就不能使用序列號了，就必須使用其它替代方案，此處不做展開說明！
總結(jié)
以上所述是小編給大家介紹的分享ORACLE SEQUENCE跳號總結(jié)，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對腳本之家網(wǎng)站的支持！
您可能感興趣的文章:- 詳解ORACLE SEQUENCE用法
- Oracle中使用觸發(fā)器(trigger)和序列(sequence)模擬實現(xiàn)自增列實例
- sqlserver實現(xiàn)oracle的sequence方法
- Oracle創(chuàng)建自增字段--ORACLE SEQUENCE的簡單使用介紹
- oracle sequence語句重置方介紹
- Hibernate Oracle sequence的使用技巧
- 通過實例了解Oracle序列Sequence使用方法
