l 立即被T1所重用
l 当T1 commit后被其它需要空闲块的事务重用，过程举例如下:
2.5 HMW概念
HIGH WATER MARK代表一个表使用的最大的(top limit)块 。2.1中已经提到HIGH WATER MARK 记录在segment header中，并且在Oracle插入数据时一般增长5个blocks（并非总是5个块，具体参见2.4.2中流程图中HMW增长方式）。
segment header block中与HWM相关信息说明如下:
| EXTENT CONTROL:
Extent Header:: spare1: 0 space2: 0 #extents: 13 #blocks: 1429 last map 0x00000000 #maps: 0 offset: 4128 Highwater:: 0x020004d0 ext#: 12 blk#: 275 ext size: 475 #blocks in seg. hdr’s freelists: 5 #blocks below: 1229 mapblk 0x00000000 offset: 12 Unlocked ==> spare1: this field is no longer used (old inc#, now always 0) ==> 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 |
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