oracle lob 简单介绍

何为LOB？
lob为oracle数据库的一个大对象数据类型,可以存储超过4000bytes的字符串，二进制数据，OS文件等大对象信息.最大可存储的容量根oracle的版本和oracle 块大小有关.
有那几种可供选择的LOB类型?
目前ORACLE提供了CLOB，NCLOB，BLOB，BFILE共四种LOB类型,CLOB,NLOB为大字符串类型,NLOB为多语言集字符类型,类似于NVARCHAR类型,用他们代替以前的LONG类型;BLOG为大二进制类型,用来代替以前的LONG RAW类型;BFILE可存储操作系统中的各种文件.
何为Internal LOBs 、External LOBs？
Internal LOBs指的是LOB数据存储在oracle的tablespace中,CLOB，NCLOB，BLOG都是Internal LOBs; 而External LOBs指的是LOB数据存储在数据库外部的操作系统文件中,BFIEL是唯一的External LOBs.BFILE提供了让我们可以在SQL中访问外部文件的方法.
LOB到底能存多大的数据呢?
来自于10G Oracle Database reference
BFILE ：Maximum size: 4 GB  Maximum size of a file name: 255 characters  Maximum size of a directory name: 30
          characters Maximum number of open BFILEs: The maximum number of BFILEs is limited by the value of the
          SESSION_MAX_OPEN_FILES initialization parameter, which is itself limited by the maximum number of open           files the operating system will allow.
BLOB： Maximum size: (4 GB - 1) * DB_BLOCK_SIZE initialization parameter (8 TB to 128 TB)  The number of LOB columns
         per table is limited only by the maximum number of columns per table (that is,10001).
CLOB： Maximum size: (4 GB - 1) * DB_BLOCK_SIZE initialization parameter (8 TB to 128 TB) The number of LOB columns
         per table is limited onlyby the maximum number of columns per table (that is,10001).
NCLOB ：Maximum size: (4 GB - 1) * DB_BLOCK_SIZE initialization parameter (8 TB to 128 TB) The number of LOB columns
  per table is limited only by the maximum number of columns per table (that is,10001).