Oracle首选列长度

Question

列长度​​的倍增因子是否会以某种方式影响数据库性能？

换句话说,以下两个表的性能有什么区别:

TBL1:
  - CLMN1 VARCHAR2(63)
  - CLMN2 VARCHAR2(129)
  - CLMN3 VARCHAR2(250)

和

TBL2:
  - CLMN1 VARCHAR2(64)
  - CLMN2 VARCHAR2(128)
  - CLMN3 VARCHAR2(256)

我们是否应该总是尝试将列的长度设置为某个功率,2或者只考虑最大值？

一些开发人员声称数据库中列长度的倍增因子之间存在某种联系,因为它影响Oracle如何分配和保存磁盘上的数据并在内存中共享其缓存.有人可以证明或反驳这一点吗？

Answer 1

性能没有区别.并且由于2的幂,没有隐藏的优化.

唯一确实存储事物的方法是实际数据.存储在VARCHAR2(2000)列中的100个字符的存储方式与存储在VARCHAR2(500)列中的100个字符完全相同.

将长度视为业务约束,而不是数据类型的一部分.唯一可以推动您决定长度的因素是关于数据的业务限制.

编辑:长度确实产生影响的唯一情况是,当您需要该列的索引时.较旧的Oracle版本(<10)确实对密钥长度有限制,并在创建索引时进行了检查.

即使在Oracle 11中可行,但对于具有4000个字符的值具有索引可能不是最明智的选择.

编辑2:

所以我很好奇并设置了一个简单的测试:

create table narrow (id varchar(40));
create table wide (id varchar(4000));

然后在两个表中填充由40'X'组成的字符串.如果存储之间确实存在(实质性)差异,那么在检索数据时应该以某种方式显示,对吧？

两个表都有1048576行.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> set autotrace traceonly statistics
SQL> select count(*) from wide;


Statistics
----------------------------------------------------------
          0  recursive calls
          1  db block gets
       6833  consistent gets
          0  physical reads
          0  redo size
        349  bytes sent via SQL*Net to client
        472  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> select count(*) from narrow;


Statistics
----------------------------------------------------------
          0  recursive calls
          1  db block gets
       6833  consistent gets
          0  physical reads
          0  redo size
        349  bytes sent via SQL*Net to client
        472  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL>

因此,两个表的全表扫描完全相同.那么当我们实际选择数据时会发生什么？

SQL> select * from wide;

1048576 rows selected.


Statistics
----------------------------------------------------------
          4  recursive calls
          2  db block gets
      76497  consistent gets
          0  physical reads
          0  redo size
   54386472  bytes sent via SQL*Net to client
     769427  bytes received via SQL*Net from client
      69907  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
    1048576  rows processed

SQL> select * from narrow;

1048576 rows selected.


Statistics
----------------------------------------------------------
          4  recursive calls
          2  db block gets
      76485  consistent gets
          0  physical reads
          0  redo size
   54386472  bytes sent via SQL*Net to client
     769427  bytes received via SQL*Net from client
      69907  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
    1048576  rows processed

SQL>

一致性获取略有不同,但这可能是由于缓存造成的.