如何为Oracle的CLOB类型解释ALL_TAB_COLS.DATA_LENGTH = 4000?

问题描述

当我创建一个包含CLOB的表时，报告CLOB列的DATA_LENGTH为4000:

When I create a table with a CLOB in it, the CLOB column is reported to have a DATA_LENGTH of 4000:

create table test (
  data clob
);

-- Returns 4000
select data_length from all_tab_cols
where table_name = 'TEST';

我该怎么解释?考虑到VARCHAR2的限制也是4000，这是否是一些向后兼容的调整?还是这表示我只能插入/更新长度为4000的字符串文字?是否在某处记录了 SYS.ALL_TAB_COLS 的这种行为? /p>

How can I interpret that? Is that some backwards-compatible tweak, considering that the limit for VARCHAR2 is also 4000? Or is this to indicate that I can only insert / update string literals of length 4000? Is this behaviour of SYS.ALL_TAB_COLS documented somewhere?

推荐答案

表空间中最多可以在线存储4000字节.如果CLOB的长度超过4000字节，则必须通过特殊功能将其存储在LOB存储区中或从中读取.

Up to 4000 bytes can be stored in-line in the tablespace. If the length of the CLOB exceeds 4000 bytes it must be stored in/readed from LOB storage area via special functions.

另请参阅:

• http://www.dba-oracle.com/t_blob.htm
• http://www.dba-oracle.com/t_table_blob_lob_storage.htm

• http://www.dba-oracle.com/t_blob.htm
• http://www.dba-oracle.com/t_table_blob_lob_storage.htm

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