allocation_type中的三个值有什么区别？

fish617

在dba_tablespaces中，有个字段叫allocation_type，有三种不同的值，分别为user,system,uniform。这三个值有什么区别啊？
各位大虾，请赐教！

fish617

没有理，我顶！

fish617

怎么没人理睬，高手呢？
快帮帮小弟，谢谢

bigbombgj

Allocation Types in LMTs

Allocation type plays a very important role in how the LMT is behaving. It specifies how the extent is being allocated by the system. There are three types of allocating extents in LMTs- USER, SYSTEM and UNIFORM.

    USER- The LMT behaves as DMT, allocating extents as per the storage clause provided with the object or defaulted at tablespace level. The advantage is that allocation of extents is managed at the datafile level and such tablespaces will not compete for ST enqueue. The disadvantage is that such tablespaces are not subject to uniform extent allocation policy. DMTs that are converted to LMTs fall under this type.

    SYSTEM- Oracle manages the space. The extents are auto allocated by the system based on an internal algorithm. Allocation of extents is managed at the datafile level and such tablespaces will not compete for ST enqueue. Such tablespaces would have extents of varying sizes and would result in fragmentation and some space being wasted. This is a good alternative if the extent sizes of the various objects to be placed in the tablespace cannot be determined.

    UNIFORM- All extents are of fixed size in the system. The size is provided when creating the LMT. This type gives all the benefits offered by LMT and one should aim at achieving this.