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

blob.png

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

blob.png

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。