Data row before and after vardecimal storage format

Paul Randal in one of his earlier BLOGs described DBCC Page paul-tells-all

and the record layout. I thought it will be interesting to show how a row looks before and after the Vardecimal storage format is enabled. So here it is

Let us take a simple table

createtable t_simple (c1 char(5), c2 decimal(38,2))

go

insertinto t_simple values('aaaaa', 1.0)

go

If you run the command DBCC Page with option 3, you will get the following output

….

Slot 0 Offset 0x60 Length 29

Record Type = PRIMARY_RECORD         Record Attributes =  NULL_BITMAP    

Memory Dump @0x44D0C060

00000000:   10001a00 61616161 61016400 00000000 †....aaaaa.d.....        

00000010:   00000000 00000000 00000200 fc††††††††.............

Key things to note here is that row length is 29 bytes computed as follows

• Record Header = 4 bytes
• Column C1 = 5 bytes
• Null bit map and column count = 3 bytes
• Fixed length decimal value = 17 bytes

Now, let us enable Vardecimal storage format on this table. The following shows the row in the new storage format

Slot 0 Offset 0x60 Length 18

Record Type = PRIMARY_RECORD         Record Attributes =  NULL_BITMAP VARIABLE_COLUMNS

Memory Dump @0x44E8C060

00000000:   30000900 61616161 610200fc 01001200 †0...aaaaa.......        

00000010:   c019†††††††††††††††††††††††††††††††††..                      

Slot 0 Column 0 Offset 0x4 Length 5

c1 = aaaaa                          

Slot 0 Column 1 Offset 0x10 Length 2 [VarDecimal/VarNumeric]

c2 = 1.00                        

Note, now the row length is 18 bytes. So the size of the row is reduced from 29 bytes to 18 bytes representing a reduction in the size of the row of around 30%. Couple of other interesting points

• Decimal value is now stored in variable length portion of the record. The value is represented as ‘c019’, which is just 2 bytes.
• Since C2 now becomes the first variable length column, you see an overhead of 4 bytes for storing variable length column count (2 bytes) and offset array (2 bytes)

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