alter table参数问题

soccer

ALTER TABLE BIGTAB DEALLOCATE UNUSED KEEP 1707M

deallocate unused keep 这个参数是什么意思


谢谢

dancingpig

deallocate 是和allocate对应的
allocate是给某个segment分配extent，这里表也是一种segment
deallocate对应的就是回收分配了的extent
unused keep最简单的理解，就是回收不用的空间但保留多少
下面的是最官方的解释：
KEEP integer
Specify the number of bytes above the high water mark that the segment of the database object is to have after deallocation.

If you omit KEEP and the high water mark is above the size of INITIAL and MINEXTENTS, then all unused space above the high water mark is freed. When the high water mark is less than the size of INITIAL or MINEXTENTS, then all unused space above MINEXTENTS is freed.
If you specify KEEP, then the specified amount of space is kept and the remaining space is freed. When the remaining number of extents is less than MINEXTENTS, then MINEXTENTS is adjusted to the new number of extents. If the initial extent becomes smaller than INITIAL, then INITIAL is adjusted to the new size.
In either case, Oracle sets the value of the NEXT storage parameter to the size of the last extent that was deallocated.