Index rows containing variable-length columns require more overhead than index rows containing only fixed-length values. Use the first formula if all the keys are fixed length. Use the second formula if the keys include variable-length columns or allow NULL values.
The clustered index in the example has only fixed-length keys.
Formula |
Example |
||||
|---|---|---|---|---|---|
5 |
(Overhead) |
5 |
|||
+ |
Sum of bytes in the fixed-length index keys |
+ |
4 |
||
= Clustered row size |
9 |
||||
5 |
(Overhead) |
|
+ |
Sum of bytes in the fixed-length index keys |
|
+ |
Sum of bytes in variable-length index keys |
|
= Subtotal |
||
|
|
|
+ |
|
(Subtotal / 256) + 1 (Overhead) |
+ |
|
Number of variable-length columns + 1 |
+ |
2 |
(Overhead) |
= Clustered index row size |
The results of the division (Subtotal / 256) are rounded down.
