SQL Server数据恢复准备之TRUNCATE TABLE理解

SQL Server数据恢复准备之TRUNCATE TABLE理解

转自：https://blog.51cto.com/aimax/2142553

易语随风去关注0人评论6717人阅读2018-07-14 17:00:36

 

当truncate table发生时，如何进行恢复，相信大部分人都会选择通过还原备份到truncate table前，然后将数据重新导入正式表中。

那么在SQL Server中是不是真的只有这种方法呢，当然不是，这也是本文即将介绍的内容，前提条件是SQL Server完整恢复模式(大容量日志模式未做测试，暂不介绍)。

首先，了解一下truncate table的一些相关知识

官档：

TRUNCATE TABLE 通过释放用于存储表数据的数据页删除数据，且仅在事务日志中记录页释放。

这句话包含的信息量是很大的，通过实验进行验证，解释如下：

truncate table时，数据库日志中不会记录确切的已删除值，只记录截断记录的页的ID，并且这些记录所占用的空间会被标识为可重写，

mdf中会暂时保留这些页内的数据，当有新事务写入这些页时，truncate table的数据将会被覆盖(数据页被format，然后重新使用)。

测试：

创建一张表，并插入数据

create table test_truncate(id int,name varchar(20),address varchar(20))
go
insert into test_truncate select 1,'zhangsan','first road'
go
insert into test_truncate select 2,'wangxiao','second road'
go

DBCC TRACEON(3604):必须开启此标识才能使用

利用dbcc ind找到该表的数据页，如下PageType=1为数据页，即为：288

使用dbcc page查看数据页内容

PAGE: (1:288)
BUFFER:
BUF @0x000000000563C600
bpage = 0x0000000150020000          bhash = 0x0000000000000000          bpageno = (1:288)
bdbid = 9                           breferences = 0                     bcputicks = 0
bsampleCount = 0                    bUse1 = 56673                       bstat = 0x10b
blog = 0x7adb21cc                   bnext = 0x0000000000000000
PAGE HEADER:
Page @0x0000000150020000
m_pageId = (1:288)                  m_headerVersion = 1                 m_type = 1
m_typeFlagBits = 0x0                m_level = 0                         m_flagBits = 0x8000
m_objId (AllocUnitId.idObj) = 489   m_indexId (AllocUnitId.idInd) = 256
Metadata: AllocUnitId = 72057594069975040
Metadata: PartitionId = 72057594062241792                                Metadata: IndexId = 0
Metadata: ObjectId = 935674381      m_prevPage = (0:0)                  m_nextPage = (0:0)
pminlen = 8                         m_slotCnt = 2                       m_freeCnt = 8021
m_freeData = 167                    m_reservedCnt = 0                   m_lsn = (49:7380:2)
m_xactReserved = 0                  m_xdesId = (0:0)                    m_ghostRecCnt = 0
m_tornBits = 0                      DB Frag ID = 1
Allocation Status
GAM (1:2) = ALLOCATED               SGAM (1:3) = ALLOCATED
PFS (1:1) = 0x61 MIXED_EXT ALLOCATED  50_PCT_FULL                        DIFF (1:6) = CHANGED
ML (1:7) = NOT MIN_LOGGED
Slot 0 Offset 0x60 Length 35
Record Type = PRIMARY_RECORD        Record Attributes =  NULL_BITMAP VARIABLE_COLUMNS
Record Size = 35
Memory Dump @0x000000006DDF8060
0000000000000000:   30000800 01000000 03000002 00190023 007a6861  0..............#.zha
0000000000000014:   6e677361 6e666972 73742072 6f6164             ngsanfirst road
Slot 0 Column 1 Offset 0x4 Length 4 Length (physical) 4
id = 1
Slot 0 Column 2 Offset 0x11 Length 8 Length (physical) 8
name = zhangsan
Slot 0 Column 3 Offset 0x19 Length 10 Length (physical) 10
address = first road
Slot 1 Offset 0x83 Length 36
Record Type = PRIMARY_RECORD        Record Attributes =  NULL_BITMAP VARIABLE_COLUMNS
Record Size = 36
Memory Dump @0x000000006DDF8083
0000000000000000:   30000800 02000000 03000002 00190024 0077616e  0..............$.wan
0000000000000014:   67786961 6f736563 6f6e6420 726f6164           gxiaosecond road
Slot 1 Column 1 Offset 0x4 Length 4 Length (physical) 4
id = 2
Slot 1 Column 2 Offset 0x11 Length 8 Length (physical) 8
name = wangxiao
Slot 1 Column 3 Offset 0x19 Length 11 Length (physical) 11
address = second road
DBCC execution completed. If DBCC printed error messages, contact your system administrator.

 

可以看到数据页中存在的记录

Slot 0 Column 1 Offset 0x4 Length 4 Length (physical) 4
id = 1
Slot 0 Column 2 Offset 0x11 Length 8 Length (physical) 8
name = zhangsan
Slot 0 Column 3 Offset 0x19 Length 10 Length (physical) 10
address = first road  

Slot 1 Column 1 Offset 0x4 Length 4 Length (physical) 4
id = 2
Slot 1 Column 2 Offset 0x11 Length 8 Length (physical) 8
name = wangxiao
Slot 1 Column 3 Offset 0x19 Length 11 Length (physical) 11
address = second road

 

执行truncate table后

可以看到数据页中仍保留着truncate table的相关记录。

那么，truncate table的第二种恢复方法就显而易见了，在这些数据被覆盖之前，从页面中将数据提取出来，并恢复到表中。

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