从不sequential的sequence

       遇到过好多问题关于如何在ORACLE 中创建类似SQLSERVER或ACCESS中自增长字段。答案多是先建立一个Sequence，然后在Trigger中将Sequence的NEXTVAL的取值赋予所需要的列。看上去还不错。

       

        但是一切真的那么顺利吗？Sequence 真的可以做到提供一序列连续没有遗漏的序列数值吗？

         不妨作个实验：

 

SQL> create sequence test_seq start with 1;

Sequence created.

SQL> create table test_tab ( x int) ;

Table created.

SQL> insert into test_tab values (test_seq.nextval) ;

1 row created.

SQL> insert into test_tab values (test_seq.nextval) ;

1 row created.

SQL> insert into test_tab values (test_seq.nextval) ;

1 row created.

SQL> commit;

Commit complete.

SQL> select * from test_tab ;

X
----------
1
2

3

SQL> conn / as sysdba;
Connected.
SQL> alter system flush shared_pool ;

System altered.

SQL> conn user1/user1
Connected.
SQL> insert into test_tab values (test_seq.nextval) ;

1 row created.

SQL> commit;

Commit complete.

SQL> select * from test_tab ;

X
----------
1
2

3
21

 

 

         从试验中可以看出，在缺省情况下，我们建立的是带有Cache选项的Sequence (缺省值是20), 它的作用是预先将一定数量的序列值存放在SGA中，便于快速访问。可是它的副作用就是这部分数值可能会被清除, 当下一次获取NEXTVAL时，就会不可避免地造成序列值丢失。

         总结一下，在以下情况下，序列值会丢失：

 

1．  数据库关闭或重起 ，由于整个SGA会被清除，所以Cached的序列值同样会被清除。

2.        类似于普通的Data Block ,当SGA中需要放置新的数据，Cached的序列值可能会按照SGA的数据存放规则被清除。

 

读到这里，细心的读者也许会问，如果在创建Sequence时，有意不选用Cache选项，问题不就解决了吗？且慢，还有两点需要注意：

 

1． 访问效率降低，没有Cache功能的Sequence取值将无法直接访问内存

2． 不论是Nocache还是Cache , 每次访问NEXTVAL的过程都是不可逆的，在同一session中，在执行一系列DML和Sequence的操作后，用户执行Rollback,希望将操作回滚，但是Sequence此时就显得异常顽固，用掉的NEXTVAL将无法被重现。当下一次试图读取NEXTVAL时，Sequence的指针又移动到下一位了。

 

        看来Oracle真是一个海洋，每个细小的知识点都是那么饶有趣味，值得我们去努力专研啊。

 

 

备注：使用Cache功能对Sequence读取效率的影响

Connected to:

Oracle8i Enterprise Edition Release 8.1.7.4.1 - Production

With the Partitioning option

JServer Release 8.1.7.4.1 - Production

 

SQL> set timing on

SQL> set autotrace traceonly statistics

SQL> SELECT * FROM ALL_OBJECTS;

14302 rows selected.

Elapsed: 00:00:13.05

Statistics

----------------------------------------------------------

          7  recursive calls

          4  db block gets

     146635  consistent gets

          0  physical reads

          0  redo size

    1633344  bytes sent via SQL*Net to client

     117520  bytes received via SQL*Net from client

        956  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

      14302  rows processed

 

SQL>

SQL> -- 测试带有CACHE选项的Sequence:

SQL>

SQL> CREATE SEQUENCE test_seq1 CACHE 1000;

 

Sequence created.

 

Elapsed: 00:00:00.00

SQL> SELECT x.*, test_seq1.NEXTVAL FROM ALL_OBJECTS x;

 

14303 rows selected.

 

Elapsed: 00:00:13.09

 

Statistics

----------------------------------------------------------

        202  recursive calls

         64  db block gets

     146636  consistent gets

          0  physical reads

      10468  redo size

    1752002  bytes sent via SQL*Net to client

     117543  bytes received via SQL*Net from client

        956  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

      14303  rows processed

 

SQL>

SQL> --测试不带有CACHE选项的Sequence

SQL>

SQL> DROP SEQUENCE test_seq1;

 

Sequence dropped.

 

Elapsed: 00:00:00.00

SQL> CREATE SEQUENCE test_seq1 NOCACHE;

 

Sequence created.

 

Elapsed: 00:00:00.00

SQL> SELECT x.*, test_seq1.NEXTVAL FROM ALL_OBJECTS x;

 

14303 rows selected.

 

Elapsed: 00:00:32.02        （执行时间明显长了）

 

Statistics

----------------------------------------------------------

     185946  recursive calls

      57216  db block gets

     160925  consistent gets

          0  physical reads

   10004008  redo size

    1752002  bytes sent via SQL*Net to client

     117543  bytes received via SQL*Net from client

        956  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

      14303  rows processed



 

       

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原文转自：http://www.ltesting.net