SQL Server Tacklebox- P43 docx

Figure 8.4 shows the output from rerunning DBCC PAGE, with this trace flag turned on.. Recall, this is a database with no tables or any other objects created and with no data inserted..

Figure 8.4 shows the output from rerunning DBCC PAGE, with this trace flag turned on

Figure 8.4: DBCC PAGE with trace flag 3604 turned on

At the bottom of the output I can see that pages 1:172 – 1:383 are not allocated, and all pages are 0% full Recall, this is a database with no tables or any other objects created and with no data inserted

So, let's now create a simple table and insert some data into it The script to do this in is shown in Listing 8.1 It creates a table in the NEO database, called ONE, and inserts into it 1000 records (well, 999 really) Simple stuff, but the important point in the context of this example is that this data load will cause additional pages to be allocated to the database and be filled with data, and I'll be able to

8 – Finding data corruption

USE [NEO]

GO

IF EXISTS SELECT FROM sys.objects WHERE object_id OBJECT_ID ( '[dbo].[ONE]') AND type in N'U'))

DROP TABLE [dbo].[ONE]

GO

CREATE TABLE [dbo].[ONE](

[NEOID] [int] NULL,

[NEOTEXT] [nchar](50) NULL

) ON [PRIMARY]

GO

BEGIN Tran T_Time

DECLARE @SQL_Alphabet varchar(26)

SET @SQL_Alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

DECLARE @rnd_seed int

SET @rnd_seed = 26

DECLARE @counter int 1

WHILE @counter < 1000

BEGIN

Insert Into ONE

Values (

@counter,

(select SUBSTRING (@SQl_alphabet,

Cast ( RAND () @rnd_seed as int) + 1, CAST ( RAND () @rnd_seed as int) + 1) )

)

SET @counter = @counter + 1

END

Commit Tran T_Time

Listing 8.1 Creating and populating the ONE table

Figure 8.5 shows the sample data that was inserted

Figure 8.5: Sample data in the ONE table

From Figure 8.4, I already know that, for our empty database, pages 1:172 – 1:383 were unallocated Re-running DBCC PAGE should reveal that more pages have been allocated to accommodate this data, and that those pages have different percentages of fullness Figure 8.6 shows the new results

8 – Finding data corruption

Figure 8.6: New Pages added to NEO database after loading data

I can see that pages 1:184 – 1:189, for example, are now allocated and are 100 percent full Having identified one of the new pages (1:184) that contains the data that I just loaded, I can run DBCC PAGE again for that specific page and return a basket full of information, as shown in Figure 8.7

Figure 8.7 Individual records from page 1:184

I can see, for example, that it returns the actual value for both NEOID and

NEOTEXT, 553 and UVWXYZ respectively It also returns a hex dump (10006c00 29020000…) that specifies the specific location in the data file where the record with NEOID 533 is stored

If you are not an expert in reading hexadecimal then fear not; neither am I at this point I do know, however, that using this information I will be able to find this exact same record and modify it outside of SQL Server, which will really wreak some havoc For that however, I will need my trusty hexadecimal editor, which I will discuss shortly