an>
==> space2: this field is no longer used (old ts#, now always 0)==> #extents: number of extents allocated to segment==> #blocks: number of blocks allocated to segment ==> last map: address of last extent map block 0 if extent map is entirely in the segment header==> #maps: number of extent map block==> offset: offset to end of extent map ==> hwm dba: address of block at highwater mark==> ext#: hwm extent number relative to segment==> blk#: hwm block number within extent==> ext size: hwm extent size (in blocks)==> #blocks in seg. hdr's freelists: number of blocks in seg. hdr's free list ==> #blocks below: number of blocks below hwm==> mapblk dba: dba of extent map block containing hwm extent is 0 if hwm is in the segment header==> offset: offset within extent map block is the ext# if hwm is in segment header==> locked by: if locked by a transaction, the xid is displayed
hwm可以说是已经使用过的存储空间和未使用过的存储空间之间的分界线。在表使用过程中，hwm一直向一个方向移动，插入记录时hwm可能会向增加的方向移动，但是删除记录时hwm并不会向相反的方向移动。参见2.4.2。下图显示了某个数据段中hwm的位置情况。
图2.5
