How I Rescue Data From Corrupted Databases

Edit: Some changes added

This is my procedure for "rescuing" data from a corrupted database. Obviously restoring from backup is a lot easier!

0) Set the damaged database to Read-Only. if you don't have a backup make one now.

1) Script the database

2a) Create a new, TEMP database - preferably on a different machine in case of hardware problems on the original machine

2b) Size the Data for the TEMP database same size as the original (to avoid dynamic extensions). Size the Log something large-ish!

3) Run the Script on the TEMP database. Do NOT create any FK etc. yet

4a) Attempt to transfer all tables:

-- Prepare script of: INSERT INTO ... SELECT * FROM ...
SET NOCOUNT ON
SELECT 'PRINT ''' + name + '''' + CHAR(13) + CHAR(10) + 'GO' + CHAR(13) + CHAR(10)
+ CASE WHEN C.id IS NULL
THEN ''
ELSE 'SET IDENTITY_INSERT dbo.[' + name + '] ON' + CHAR(13) + CHAR(10)
END
+ 'INSERT INTO MyTempDatabase.dbo.[' + name + ']' + CHAR(13) + CHAR(10)
+ 'SELECT * FROM dbo.[' + name + ']' + CHAR(13) + CHAR(10)
+ CASE WHEN C.id IS NULL
THEN ''
ELSE 'SET IDENTITY_INSERT dbo.[' + name + '] OFF' + CHAR(13) + CHAR(10)
END
+ 'GO'
FROMdbo.sysobjects AS O
LEFT OUTER JOIN
(
SELECT DISTINCT C.id
FROMdbo.syscolumns AS C
WHEREC.colstat = 1-- Identity column
) AS C
ON C.id = O.id
WHERE type = 'U'
AND name NOT IN ('dtproperties')
ORDER BY name
SET NOCOUNT OFF

this generates statements like this:

PRINT 'MyTable'
GO
SET IDENTITY_INSERT dbo.[MyTable] ON
INSERT INTO RESTORE_XFER_TEMP.dbo.[MyTable]
SELECT * FROM dbo.[MyTable]
SET IDENTITY_INSERT dbo.[MyTable] OFF
GO

4b) This will give some sort of error on the tables which cannot be copied, and they will need to be rescued by some other means.

5a) Each "broken" table needs to be rescued using an index. Ideally you will have a clustered index on the PK and that will be undamaged, so you can "rescue" all the PKs into a temp table:

-- Copy PK fields to a temporary table
-- DROP TABLE MyRestoreDatabase.dbo.TEMP_RESCUE_PK
-- TRUNCATE TABLE MyRestoreDatabase.dbo.MyBrokenTable
SELECT[ID]=IDENTITY(int, 1, 1),
[IsCopied]=CONVERT(tinyint, 0),
MyPK
INTOMyRestoreDatabase.dbo.TEMP_RESCUE_PK
FROMMyBrokenDatabase.dbo.MyBrokenTable
ORDER BY MyPK

5b) If that is successful you have a list of all the PKs, so can can try to copy data matching those PKs, in batches:

-- If OK then selectively copy data across
-- First Prep. a temp Batch table
-- DROP TABLE MyRestoreDatabase.dbo.TEMP_RESCUE_BATCH
SELECT TOP 1 [ID]=CONVERT(int, NULL), [IsCopied]=CONVERT(bit, 0), MyPK
INTOMyRestoreDatabase.dbo.TEMP_RESCUE_BATCH
FROMMyRestoreDatabase.dbo.TEMP_RESCUE_PK
GO
--
DECLARE@intStartint,
@intStopint,
@intBatchSizeint

-- NOTE: After the first run set these to any "gaps" in the table that you want to fill
SELECT
@intStart = 1,
@intBatchSize = 10000,
@intStop = (SELECT MAX([ID]) FROM MyRestoreDatabase.dbo.TEMP_RESCUE_PK)

SELECT@intStart = MIN([ID])
FROMMyRestoreDatabase.dbo.TEMP_RESCUE_PK
WHERE IsCopied = 0
AND [ID] >= @intStart

WHILE@intStart < @intStop
BEGIN
SET ROWCOUNT @intBatchSize

-- Isolate batch of Keys into separate table
TRUNCATE TABLE MyRestoreDatabase.dbo.TEMP_RESCUE_BATCH
INSERT INTO MyRestoreDatabase.dbo.TEMP_RESCUE_BATCH
SELECTT.*
FROMMyRestoreDatabase.dbo.TEMP_RESCUE_PK AS T
WHERE IsCopied = 0
AND [ID] >= @intStart
AND [ID] < @intStart + @intBatchSize

-- Attempt to copy matching records, for this batch
PRINT CONVERT(varchar(20), @intStart)
INSERT INTO MyRestoreDatabase.dbo.MyBrokenTable
SELECTS.*
FROMMyRestoreDatabase.dbo.TEMP_RESCUE_BATCH AS T
LEFT OUTER JOIN MyRestoreDatabase.dbo.MyBrokenTable AS D
ON D.MyPK = T.MyPK
-- This will try to get the data from the broken table, it may fail!
JOIN MyBrokenDatabase.dbo.MyBrokenTable AS S
ON S.MyPK = T.MyPK
WHERED.MyPK IS NULL-- Belt and braces so as not to copy existing rows

-- Flag the rows just "Copied"
UPDATEU
SETIsCopied = 1
FROMMyRestoreDatabase.dbo.TEMP_RESCUE_PK AS U
WHEREIsCopied = 0
AND [ID] >= @intStart
AND [ID] < @intStart + @intBatchSize

-- Loop round, until done
SELECT@intStart = @intStart + @intBatchSize
END
GO
SET ROWCOUNT 0-- Turn OFF!!
GO

5c) This will copy in batches of 10,000 [you can adjust @intbatchSize depending on table size] until it gets to a damaged part of the table, then it will abort.

Change the @intStart to the last ID number displayed, and reduce @intBatchSize (by an order of magnitude each time) until you have rescued as many records as possible in the first "part" of the table.

5d) Reset the batch size @intBatchSize to 10,000 [or whatever size is appropriate], and increase the @intStart repeatedly until you are past the damaged section - copying will start again, and will abort if there are further damaged sections

5e) Repeat that process until you have rescued as much of the data as is possible

6) Check what is left to be rescued

-- Check amount NOT done:
SELECTCOUNT(*), MIN([ID]), MAX([ID])
FROMMyRestoreDatabase.dbo.TEMP_RESCUE_PK
WHERE IsCopied = 0
--AND [ID] > 123456-- Optionally count items after a "gap"
--
-- Double check that IsCopied set correctly, and the number of records "lost"
SELECTCOUNT(*),
[IsCopied] = SUM(CONVERT(int, IsCopied)),
[IsCopied+Record] = SUM(CASE WHEN IsCopied = 1 AND C.MyPK IS NOT NULL THEN 1 ELSE 0 END),
[IsCopiedNoRecord] = SUM(CASE WHEN IsCopied = 1 AND C.MyPK IS NULL THEN 1 ELSE 0 END),
[IsNOTCopied] = SUM(CASE WHEN IsCopied = 0THEN 1 ELSE 0 END),
[IsNOTCopied+Record] = SUM(CASE WHEN IsCopied = 0 AND C.MyPK IS NOT NULL THEN 1 ELSE 0 END),
[IsNOTCopiedNoRecord] = SUM(CASE WHEN IsCopied = 0 AND C.MyPK IS NULL THEN 1 ELSE 0 END)
FROMMyRestoreDatabase.dbo.TEMP_RESCUE_PK AS T
LEFT OUTER JOIN MyRestoreDatabase.dbo.MyBrokenTable AS C
ON C.MyPK = T.MyPK
--
-- List of "Lost" records
SELECTMyPK
FROMMyRestoreDatabase.dbo.TEMP_RESCUE_PK
WHERE IsCopied = 0
ORDER BY [ID]

You will then have to "find" and rescue the lost records somewhere.

I have a further process using OPENQUERY() to rescue records to fill the gaps in the event that they are available on a remote system - a straight JOIN to get them is going to be far to slow on anything other than tiny tables!

7a) Create the FKs etc. Update the statistics, rebuild indexes, and possibly shrink the Log if it is unrealistically big
7b) Backup and Restore over the original database
7c) DBCC CHECKDB ('MyDatabaseName') WITH ALL_ERRORMSGS, NO_INFOMSGS

Good luck!

Kristen