对序列进行迁移时产生间断的原因

我们知道创建序列时，会有默认的或者指定的 n个值会cache到内存中，当数据库所在的服务器down 机或者迁移序列时，会发生序列丢失的问题。现在就第二种进行实验。这里并没有什么方法可以解决序列 间隔的问题，如果有，也请大家给以事例。

yang@rac1>create sequence yang_seq ;

Sequence created.

yang@rac1>select yang_seq.nextval from dual;

   NEXTVAL

----------

         1

yang@rac1>/

         2

         3

         4

         5

         6

         7

导出序列：

oracle@rac1:rac1 /tmp>expdp yang/yang  directory=dumpdir dumpfile=sequence.dmp  include=sequence  

Export: Release 11.2.0.1.0 - Production on Fri Apr 1 22:59:31 2011

Copyright (c) 1982, 2009, Oracle and/or its affiliates.  All rights reserved.

Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production

With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,

Data Mining and Real Application Testing options

Starting "YANG"."SYS_EXPORT_SCHEMA_01":  yang/******** directory=dumpdir dumpfile=sequence.dmp include=sequence

Estimate in progress using BLOCKS method...

Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA

Total estimation using BLOCKS method: 0 KB

Processing object type SCHEMA_EXPORT/SEQUENCE/SEQUENCE

Master table "YANG"."SYS_EXPORT_SCHEMA_01" successfully loaded/unloaded

******************************************************************************

Dump file set for YANG.SYS_EXPORT_SCHEMA_01 is:

  /tmp/dump/sequence.dmp

Job "YANG"."SYS_EXPORT_SCHEMA_01" successfully completed at 23:01:06

删除序列

yang@rac1>drop sequence yang_seq;

Sequence dropped.

导入序列：

oracle@rac1:rac1 /tmp>impdp yang/yang  directory=dumpdir dumpfile=sequence.dmp  table_exists_action=skip include=sequence  

Import: Release 11.2.0.1.0 - Production on Fri Apr 1 23:06:21 2011

Master table "YANG"."SYS_IMPORT_FULL_01" successfully loaded/unloaded

Starting "YANG"."SYS_IMPORT_FULL_01":  yang/******** directory=dumpdir dumpfile=sequence.dmp table_exists_action=skip include=sequence

Job "YANG"."SYS_IMPORT_FULL_01" successfully completed at 23:07:21

再次验证：

        21

从7 间断了13个 （注意：默认的是20个）

从下面的实验中可以看出 我们导入sequence时 数据库都做了什么：

oracle@rac1:rac1 /tmp>impdp yang/yang  directory=dumpdir dumpfile=sequence.dmp   SQLFILE=seq.sql                                               

Import: Release 11.2.0.1.0 - Production on Fri Apr 1 23:15:16 2011

Master table "YANG"."SYS_SQL_FILE_FULL_01" successfully loaded/unloaded

Starting "YANG"."SYS_SQL_FILE_FULL_01":  yang/******** directory=dumpdir dumpfile=sequence.dmp SQLFILE=seq.sql

Job "YANG"."SYS_SQL_FILE_FULL_01" successfully completed at 23:16:13

oracle@rac1:rac1 /tmp>cd dump

oracle@rac1:rac1 /tmp/dump>ls

export.log  import.log  seq.sql  sequence.dmp  t.dmp

oracle@rac1:rac1 /tmp/dump>cat seq.sql

-- CONNECT YANG

ALTER SESSION SET EVENTS '10150 TRACE NAME CONTEXT FOREVER, LEVEL 1';

ALTER SESSION SET EVENTS '10904 TRACE NAME CONTEXT FOREVER, LEVEL 1';

ALTER SESSION SET EVENTS '25475 TRACE NAME CONTEXT FOREVER, LEVEL 1';

ALTER SESSION SET EVENTS '10407 TRACE NAME CONTEXT FOREVER, LEVEL 1';

ALTER SESSION SET EVENTS '10851 TRACE NAME CONTEXT FOREVER, LEVEL 1';

ALTER SESSION SET EVENTS '22830 TRACE NAME CONTEXT FOREVER, LEVEL 192 ';

-- new object type path: SCHEMA_EXPORT/SEQUENCE/SEQUENCE

 CREATE SEQUENCE  "YANG"."YANG_SEQ"  MINVALUE 1 MAXVALUE 9999999999999999999999999999 INCREMENT BY 1 START WITH 21 CACHE 20 NOORDER  NOCYCLE ;

每次导入队列时，会从cache 的 N+1 开始（N 为创建序列时cache 在内存中的值 ）。这就是为什么会出现间断了。