Data Page Structure in MS SQL Server 6.5

General concepts

There are five kinds of pages in MS SQL 6.5:

In this article I want to tell you about data page structure in
MS SQL 6.5. The size of the data page in MS SQL 6.5 is 2Kb, i.e.
2048 bytes. Every data page consists of three part:

See Figure 1:

Figure 1. General data page structure

This is the description of the page header:

pageno    - is a logical page number
nextpg    - next logical page number
prevpg    - previous logical page number
objid     - id of the table
timestamp - internal identifier
nextrno   - next row number, that will be written on this page
level     - the index level
indid     - id of the index
freeoff   - pointer to the free space at the end of the page
minlen    - minimum rows length for this data page

Offset table contains two bytes for each row on the data page.
MS SQL 6.5 uses the offset table to find the address of row.
Every cell of the offset table contains the address of the row
on the data page.

When you delete record, MS SQL 6.5 writes in the appropriate
cell the zero value, deletes row, moves other rows to avoid
free space between them and sets the new value into freeoff
field of the page header. All rows are stored continuously
on the data page.

When you insert new record, MS SQL 6.5 scans the offset table
from the end. If there is cell with zero value in it, then
offset of the new row will be inserted into this cell. If there
is no cell with zero values in it, then new cell will be added
in the offset table, new value will be written into freeoff field
of the page header and the nextrno field of the page header will
be increased by 1.

You can use the following command to view the data page structure
(this command is not very good documented in SQL Server Books Online):

DBCC PAGE ({dbid|dbname}, pagenum [,print option] [,cache] [,logical])

PARAMETERS:

   Dbid or dbname - Enter either the dbid or the name of the database

                    in question.

   Pagenum - Enter the page number of the SQL Server page that is to

             be examined.
   Print option - (Optional) Print option can be either 0, 1, or 2.
                  0 - (Default) This option causes DBCC PAGE to print

                      out only the page header information.

                  1 - This option causes DBCC PAGE to print out the

                      page header information, each row of information

                      from the page, and the page's offset table. Each

                      of the rows printed out will be separated from

                      each other.

                  2 - This option is the same as option 1, except it

                      prints the page rows as a single block of

                      information rather than separating the

                      individual rows. The offset and header will also

                      be displayed.
   Cache - (Optional) This parameter allows either a 1 or a 0 to be

           entered.

           0 - This option causes DBCC PAGE to retrieve the page

               number from disk rather than checking to see if it is

               in cache.

           1 - (Default) This option takes the page from cache if it

               is in cache rather than getting it from disk only.
   Logical - (Optional) This parameter is for use if the page number

             that is to be retrieved is a virtual page rather then a

             logical page. It can be either 0 or 1.
             0 - If the page is to be a virtual page number.

             1 - (Default) If the page is the logical page number.

Examples

In this examples I also use trace flag 3604.
Trace flag 3604 sends trace output to the client. This trace
flag is used only when setting trace flags with DBCC TRACEON
and DBCC TRACEOFF.

Discounts data page structure

In this example one data page is output from the table discounts,
database pubs.

USE pubs

GO

DBCC TRACEON (3604)

GO

DECLARE @pgid int

SELECT @pgid = first FROM sysindexes WHERE id = object_id('discounts')

DBCC PAGE (pubs, @pgid, 1)

GO

This is the result from my computer:

PAGE:

Page found in cache.

BUFFER:

Buffer header for buffer 0xf96580

    page=0x1181000 bdnew=0xf96580 bdold=0xf96580 bhash=0x0

bnew=0xf964c0

    bold=0xf96640 bvirtpg=2540 bdbid=5 bpinproc=0 bkeep=0 bspid=0

    bstat=0x1004   bpageno=488
PAGE HEADER:

Page header for page 0x1181000

pageno=488 nextpg=0 prevpg=0 objid=544004969 timestamp=0001 0000486a

nextrno=3 level=0 indid=0  freeoff=122 minlen=5

page status bits: 0x100,0x1
DATA:

Offset 32 -

01181020:  0100017e 041a0049 6e697469 616c2043  ...~...Initial C

01181030:  7573746f 6d657202 1707               ustomer...
Offset 58 -

0118103a:  0401019e 02200056 6f6c756d 65204469  ..... .Volume Di

0118104a:  73636f75 6e746400 e803051a 18161607  scountd.........
Offset 90 -

0118105a:  020201f4 01200043 7573746f 6d657220  ..... .Customer

0118106a:  44697363 6f756e74 38303432 031c1807  Discount8042....
OFFSET TABLE:

Row - Offset

2 (0x2) - 90 (0x5a),   1 (0x1) - 58 (0x3a),

0 (0x0) - 32 (0x20),

See Figure 2:

Figure 2. Discounts data page structure

After delete

When you delete record, MS SQL 6.5 writes in the appropriate
cell the zero value, deletes row, moves other rows to avoid
free space between them and sets the new value into freeoff
field of the page header. All rows are stored continuously
on the data page.

DELETE FROM discounts WHERE discounttype = 'Volume Discount'

This is the result from my computer:

PAGE:

Page found in cache.

BUFFER:

Buffer header for buffer 0xf96580

    page=0x1181000 bdnew=0xf5a448 bdold=0xf5a448 bhash=0x0

bnew=0xf964c0

    bold=0xf96640 bvirtpg=2540 bdbid=5 bpinproc=7 bkeep=0 bspid=0

    bstat=0x1004   bpageno=488
PAGE HEADER:

Page header for page 0x1181000

pageno=488 nextpg=0 prevpg=0 objid=544004969 timestamp=0001 0000519c

nextrno=3 level=0 indid=0  freeoff=90 minlen=5

page status bits: 0x100,0x10,0x1
DATA:

Offset 32 -

01181020:  0100017e 041a0049 6e697469 616c2043  ...~...Initial C

01181030:  7573746f 6d657202 1707               ustomer...
Offset 58 -

0118103a:  020201f4 01200043 7573746f 6d657220  ..... .Customer

0118104a:  44697363 6f756e74 38303432 031c1807  Discount8042....
OFFSET TABLE:

Row - Offset

2 (0x2) - 58 (0x3a),   1 (0x1) - 0 (0x0),

0 (0x0) - 32 (0x20),

See Figure 3:

Figure 3. After delete

After insert

When you insert new record, MS SQL 6.5 scans the offset table
from the end. If there is cell with zero value in it, then
offset of the new row will be inserted into this cell. If there
is no cell with zero values in it, then new cell will be added
in the offset table, new value will be written into freeoff field
of the page header and the nextrno field of the page header will
be increased by 1.

insert discounts values('Volume Discount', NULL, 100, 1000, 6.7)

This is the result from my computer:

PAGE:

Page found in cache.

BUFFER:

Buffer header for buffer 0xf96580

    page=0x1181000 bdnew=0xf5a448 bdold=0xf5a448 bhash=0x0

bnew=0xf964c0

    bold=0xf96640 bvirtpg=2540 bdbid=5 bpinproc=8 bkeep=0 bspid=0

    bstat=0x1004   bpageno=488
PAGE HEADER:

Page header for page 0x1181000

pageno=488 nextpg=0 prevpg=0 objid=544004969 timestamp=0001 0000519e

nextrno=3 level=0 indid=0  freeoff=122 minlen=5

page status bits: 0x100,0x10,0x1
DATA:

Offset 32 -

01181020:  0100017e 041a0049 6e697469 616c2043  ...~...Initial C

01181030:  7573746f 6d657202 1707               ustomer...
Offset 58 -

0118103a:  020201f4 01200043 7573746f 6d657220  ..... .Customer

0118104a:  44697363 6f756e74 38303432 031c1807  Discount8042....
Offset 90 -

0118105a:  0401019e 02200056 6f6c756d 65204469  ..... .Volume Di

0118106a:  73636f75 6e746400 e803051a 18161607  scountd.........
OFFSET TABLE:

Row - Offset

2 (0x2) - 58 (0x3a),   1 (0x1) - 90 (0x5a),

0 (0x0) - 32 (0x20),

See Figure 4:

Figure 4. After insert

Literature

1. SQL Server Books Online.

2. INFO: Description of DBCC PAGE Command

http://support.microsoft.com/support/kb/articles/Q83/0/65.ASP