BFILE 二进制文件,存储在数据库外的操作系统文件,只读的。把此文件当二进制处理。
BLOB 二进制大对象。存储在数据库里的大对象,一般是图像声音等文件。
CLOB 字符型大对象。一般存储大数量文本信息。存储单字节,固定宽度的数据。
NCLOB 字节字符大对象。存储单字节大块,多字节固定宽度,多字节变宽度数据
BFILE
The BFILE data type enables access to binary file LOBs that are stored in file systems outside Oracle Database. A BFILE column or attribute stores a BFILE locator, which serves as a pointer to a binary file on the server file system. The locator maintains the directory name and the filename.
通过bfile类型,oracle数据库可以访问存储在操作系统上的二进制文件。我们可以认为在bfile中存放的是指向操作系统文件的指针。使用bfile必须首先创建directory,并具有相应的权限。
You can change the filename and path of a BFILE without affecting the base table by using the BFILENAME function.
通过bfilename内置函数设置bfile类型的值,该函数具有两个参数,即directory和filename。
Binary file LOBs do not participate in transactions and are not recoverable. Rather, the underlying operating system provides file integrity and durability. BFILE data can be up to 264-1 bytes, although your operating system may impose restrictions on this
maximum.The database administrator must ensure that the external file exists and that Oracle processes have operating system read permissions on the file.The BFILE data type enables read-only support of large binary files. You cannot modify or replicate such a file. Oracle provides APIs to access file data. The primary interfaces that you use to access file data are the DBMS_LOB package and Oracle Call Interface (OCI).
对bfile指向的操作系统文件的操作,不被数据库事务管理,有操作系统来负责一致性和持久性,bfile可以处理的文件大小为2^64-1个字节,因此对bfile文件大小的限制往往来自操作系统层面。对于bfile指向的文件,数据库具有只读权限,不可以编辑和复制。我们主要通过DBMS_LOB包来管理和操作bfile类型数据。
<span style="font-size:12px">[oracle@oadata dir1]$ pwd /home/oracle/oradir/dir1 [oracle@oadata dir1]$ ls 1.txt 2.txt [oracle@oadata dir1]$ cat 1.txt 1 2 3 4 5 6 [oracle@oadata dir1]$ cat 2.txt a b c d [oracle@oadata dir1]$ sqlplus / as sysdba SQL*Plus: Release 11.2.0.3.0 Production on 星期六 10月 12 11:58:08 2013 Copyright (c) 1982, 2011, Oracle. All rights reserved. 连接到: 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> create direcotry dir1 as '/home/oracle/oradir/dir1'; create direcotry dir1 as '/home/oracle/oradir/dir1' * 第 1 行出现错误: ORA-00901: 无效 CREATE 命令 SQL> create directory dir1 as '/home/oracle/oradir/dir1'; 目录已创建。 SQL> grant read,write on directory dir1 to easy; 授权成功。 SQL> conn easy/jodezhu 已连接。 SQL> show user USER 为 "EASY" SQL> create table tab_bfile (id number,bf bfile); 表已创建。 SQL> insert into tab_bfile values(1,bfilename('DIR1','1.txt')); 已创建 1 行。 SQL> insert into tab_bfile values(2,bfilename('DIR1','2.txt')); 已创建 1 行。 SQL> commit; 提交完成。 SQL> select * from tab_bfile; ID ---------- BF ---------------------------------------------------------------------------------------------------- 1 bfilename('DIR1', '1.txt') 2 bfilename('DIR1', '2.txt') SQL> set serveroutput on DECLARE r tab_bfile%rowtype; CURSOR c IS SELECT id,bf FROM tab_bfile; AMOUNT INTEGER := 100; OFFSET INTEGER := 1; OUTRAW VARCHAR2(100); BEGIN OPEN c; LOOP FETCH c INTO r; EXIT WHEN c%notfound; dbms_output.put_line(r.id); dbms_lob.open(r.bf,DBMS_LOB.LOB_READONLY); DBMS_LOb.READ(r.bf,amount,offset,outraw); dbms_output.put_line(outraw); dbms_lob.close(r.bf); END LOOP; 21 END; 22 / 1 310A320A330A340A350A360A 2 610A620A630A640A PL/SQL 过程已成功完成。 SQL> </span>
bfile类型在使用之前,必须要先打开。
BLOB\CLOB\NCLOB
blob\clob\nclob称为内部lob(bfile称为外部lob),其大小限制为2^32-1与所在表空间数据块大小乘积,大概范围在8T到128T,因此多数情况下,我们不必担心存储上限的问题。blob类型存储的是二进制流数据,而clob和nclob存储的是大规模的字符数据,在clob中按照数据库的字符集存储,而在nclob中按照数据库的国家字符集存储。内部lob由于存储在数据库内部,因此参与数据库的事务处理,在发送错误时可以进行回滚处理。可以通过plsql dbms_lob包或者oci接口来处理lob数据。
对lob数据的处理,可以不用在open/close函数对之间运行,此时,伴随着对lob的处理,会及时更新相应的索引,这可能会影响效率。当我们将其放在open/close对中时,则在close时,对索引进行更新。open/close必须成对出现。
注意:
It is an error to commit the transaction before closing all opened LOBs that were opened by the transaction. When the error is returned, the openness of the open LOBs is discarded, but the transaction is successfully committed. Hence, all the changes made to the LOB and non-LOB data in the transaction are committed, but the domain and function-based indexes are not updated. If this happens, you should rebuild the functional and domain indexes on the LOB column.
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