Note:
Unused Block Compression is NOT used if backup done to tape using a THIRD PARTY BACKUP SOFTWARE !
Unused block compression IS executed in Standard Edition. <<<< 标准版也可以使用这种类型的压缩方式。
Skipping unused data blocks where possible enables RMAN to back up datafiles using less space, and can make I/O more efficient.
以上内容摘取自《A Complete Understanding of RMAN Compression (文档 ID 563427.1)》
以下的内容说明在10.2.0.1版本中，UNUSED BLOCK COMPRESSION压缩类型不可用：
Unused block compression is NOT available with the Base Release of 10gR2 (10.2.0.1).
It only became available in the first Patch Set Release (10.2.0.2) for NEW tablespaces only.
So for tablespaces created:
before 10.2.0.2 was applied
before compatible was set to 10.2
we cannot use Unused Block Compression.
The cause of this problem has been identified and verified in an 'unpublished' Bug 4720762 - Fixed in Product Version 11.0
以上内容摘自：《Why is RMAN not using Unused Block Compression during backup? (文档 ID 798844.1)》
总结：后经过和客户的确认，客户数据库的版本为10.2.0.4，所以我们大胆的推测，虽然客户的版本高于10.2.0.2，但是由于默认情况下数据库初始化参数COMPITABLE为空，或不符合其他条件导致UNUSED BLOCK COMPRESSION压缩类型无法在RMAN中使用到，所以凡是曾经使用到的数据块RMAN都会进行备份（NULL COMPRESSION），80GB可能是客户曾经用到过的空间大小，后来释放了相应空间，所以当前备份才呈现出40GB的数据库RMAN备份出来有80GB。
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