分享ORACLE SEQUENCE跳号总结

2020-07-26 14:04:11

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在ORACLE数据库中，序列（SEQUENCE）是使用非常频繁的一个数据库对象，但是有时候会遇到序列（SEQUECNE）跳号(skip sequence numbers)的情形，那么在哪些情形下会遇到跳号呢？

事务回滚引起的跳号

不管序列有没有CACHE、事务回滚这种情况下，都会引起序列的跳号。如下实验所示：

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----------  3SQL>

并发访问序列引起的跳号

并发访问序列引起的跳号，其实不算真正的跳号，而只是逻辑跳号，只是序列值被其它并发会话使用了。我们来构造一起并发访问序列引起的跳号，我们开启两个会话窗口，循环获取序列的值，模拟并发出现的场景。

会话窗口A：

exec dbms_lock.sleep(2); --延迟2秒执行，根据你实验情况调整/begin for i in 1 .. 2000 loop dbms_output.put_line(my_sequence.nextval); end loop;end;/

会话窗口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，我构造的实验当中，你会看到序列的跳号情况。

FLUSH SHARED_POOL会导致CACHE的序列跳号

实验测试如下所示（序列的CACHE值必须大于0），当然正常情况下，很难遇到这种情况。

SQL> select test.my_sequence.nextval from dual; NEXTVAL---------- 17004SQL> alter sequence test.my_sequence cache 40;Sequence altered.SQL> select test.my_sequence.nextval from dual; NEXTVAL---------- 17005SQL> alter system flush share_pool;alter system flush share_pool*ERROR at line 1:ORA-02000: missing SHARED_POOL/BUFFER_CACHE/GLOBAL CONTEXT keywordSQL> alter system flush shared_pool;System altered.SQL> select test.my_sequence.nextval from dual; NEXTVAL---------- 17045

数据库实例异常关闭导致跳号

如下实验所示，当数据库使用shutdown abort命令关闭后，重新启动实例，序列缓存在shared pool里面没有用过的值都没有了。一下子从17045跳到17085

SQL> select test.my_sequence.currval from dual; CURRVAL---------- 17045SQL> select object_id from dba_objects where object_name=upper('my_sequence'); OBJECT_ID---------- 97760SQL> select increment$, minvalue, maxvalue,highwater, cache 2 from seq$ where obj#=97760;INCREMENT$ MINVALUE MAXVALUE HIGHWATER CACHE---------- ---------- ---------- ---------- ----------  1  1 99999 17085  40SQL> shutdown abort;ORACLE instance shut down.SQL> startup;ORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instanceORACLE instance started.Total System Global Area 1620115456 bytesFixed Size   2213816 bytesVariable Size  1258293320 bytesDatabase Buffers  352321536 bytesRedo Buffers  7286784 bytesDatabase 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 sessionSQL> select test.my_sequence.nextval from dual; NEXTVAL---------- 17085SQL>

另外，我们也来看看正常关闭数据库的情况下，序列会不会出现跳号，我们采用10046跟踪事件，看看正常数据库关闭情况下，会对序列做一些啥操作

SQL> select test.my_sequence.nextval from dual; NEXTVAL---------- 17085SQL> 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> startupORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instanceORACLE instance started.Total System Global Area 1620115456 bytesFixed Size   2213816 bytesVariable Size  1258293320 bytesDatabase Buffers  352321536 bytesRedo Buffers  7286784 bytesDatabase 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 sessionSQL> select test.my_sequence.nextval from dual; NEXTVAL---------- 17086SQL>

Trace文件中有更新seq$数据字典表，如果你看过我这篇文章ORACLE中seq$表更新频繁的分析，基本上就知道其实seq$中维护的是序列的一些信息。通过跟踪文件，我们知道在数据库正常关闭的情况下,会触发一个update seq$的操作,把当前的sequence.nextval的值更新到seq$.highwater中,从而使得sequence在有cache的情况下，数据库正常关闭未出现nextval跳跃(currval也同样不跳跃)；而在数据库异常关闭之时，数据库不能及时将sequence.nextval更新到eq$.highwater从而引起sequence cache中的值丢失，从而可能出现了sequence使用cache导致跳跃的情况

=====================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#=:1END OF STMTPARSE #25:c=0,e=21,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=4,plh=1935744642,tim=1504236336294194BINDS #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=97760EXEC #9:c=999,e=709,p=0,cr=1,cu=2,mis=0,r=1,dep=1,og=4,plh=1935744642,tim=1504236336297033CLOSE #9:c=0,e=2,dep=1,type=3,tim=1504236336297058mealink上提到了使用dbms_shared_pool.keep将对象在锁定在shared pool 中，永远不释放。这样可以防止FLUSH SHARED POOL导致序列跳号，但是这个无法避免数据库异常关闭或CRASH引起的跳号SQL> select test.my_sequence.currval from dual; CURRVAL---------- 17086SQL> 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---------- 17086SQL> shutdown abortORACLE instance shut down.SQL> startupORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instanceORACLE instance started.Total System Global Area 1620115456 bytesFixed Size   2213816 bytesVariable Size  1258293320 bytesDatabase Buffers  352321536 bytesRedo Buffers  7286784 bytesDatabase mounted.Database opened.SQL> select test.my_sequence.nextval from dual; NEXTVAL---------- 17126