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我有一个脚本

SQL script: alter table xx_vms.es_lot_updates modify (routing_list varchar2(1000));

该列之前的大小是 255。现在我将其大小增加到 1000。但是在这个表中，routing_list 列经常是 NULL。有时，它是有价值的。

此列的增量大小是否会导致性能问题？我很困惑！请帮忙。谢谢。

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3 回答 3

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不，没有性能问题。您所做的只是允许在该列中插入更大的文本字符串。请记住，您应该始终尽可能严格地限制您希望在表格中允许的内容，您可以自由地将列设置为您需要的大小。（varchar2 最多 4000 字节）

于 2013-05-15T09:35:33.390 回答
2

这取决于。具有 varchar2(1000) 的表使用更多的数据库块进行存储，因此全表扫描可能会更昂贵，因为您要读取更多块。这是一个简单的测试：

17:48:25 HR@sandbox> create table c1000 (txt varchar2(1000));

Table created.

Elapsed: 00:00:00.21
17:48:37 HR@sandbox> create table c255 (txt varchar2(255));

Table created.

Elapsed: 00:00:00.03

-- for this test we assume that only 10% of columns are filled with data
17:50:21 HR@sandbox> ed
Wrote file S:\spool\sandbox\BUFFER_HR_32.sql

  1   insert into c255
  2   select decode(mod(rownum,10), 0, lpad('x', 255,'x')) from dual
  3*  connect by rownum <= 1e5
17:50:24 HR@sandbox> /

100000 rows created.

Elapsed: 00:00:01.26
17:50:47 HR@sandbox> ed
Wrote file S:\spool\sandbox\BUFFER_HR_32.sql

  1   insert into c1000
  2   select decode(mod(rownum,10), 0, lpad('x', 1000,'x')) from dual
  3*  connect by rownum <= 1e5
17:50:51 HR@sandbox> /

100000 rows created.

Elapsed: 00:00:01.45
17:50:52 HR@sandbox> commit;

Commit complete.

Elapsed: 00:00:00.01
17:51:20 HR@sandbox> select table_name, blocks from user_tables where table_name in ('C255', 'C1000');

TABLE_NAME                         BLOCKS
------------------------------ ----------
C1000                                1756
C255                                  622

Elapsed: 00:00:00.20
17:53:28 HR@sandbox> set autotrace traceonly statistics
17:53:37 HR@sandbox> select * from c1000;

100000 rows selected.

Elapsed: 00:00:01.30

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       1901  consistent gets
       1694  physical reads
          0  redo size
   10586458  bytes sent via SQL*Net to client
       2552  bytes received via SQL*Net from client
        201  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
     100000  rows processed

17:53:50 HR@sandbox> select * from c255;

100000 rows selected.

Elapsed: 00:00:00.63

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        825  consistent gets
        295  physical reads
          0  redo size
    3107206  bytes sent via SQL*Net to client
       2552  bytes received via SQL*Net from client
        201  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
     100000  rows processed

看consistent gets统计。与. c1000_c255

但是，如果您使用主键仅选择几行，我认为您不会注意到显着差异。

于 2013-05-15T10:04:31.817 回答
0

不，它与性能无关，varchar2 的最大大小为 4000。

一旦你改变了表，请记住编译所有依赖项，因为它们将是无效的，例如

程序和包

于 2013-05-15T10:15:22.860 回答