Fields with fixed length data types (e.g., int, money) always consume their allotted space irrespective of how much data the field actually contains. This is true even if the field is populated with a null.
Occasionally you will encounter a column in your database which is rarely used. For example, suppose you have a field called [Violation] in a table called [Employee] but very few employees have any violations – perhaps two or three for every 1000 employees. In this case, over 99% of the Violation field values are null. This would be an example of a sparsely populated field.
To demonstrate a sparsely populated field, we will create a simple table with the design shown here. Create and populate the dbo.Bonus table by running this code in the figures below.
Please note that INSERT statements demonstrated will use row constructors, which debuted as a new feature in SQL Server 2008. However, readers who are running SQL Server 2005 must write their INSERT statements using the alternate syntax shown in the lower portion of the last Figure.
Now look at all the records in the table. Since all fields contain fixed length data types (i.e., int, char(2), money), we could have accurately calculated the per row consumption before we even added any data to the table.
Each row of the Bonus table will consume 21 bytes based on the data below.
- Row Header – 4 bytes (true for all rows).
- Fixed Data – 14 bytes (BonusID is an int (4 bytes) and BonusAmount’s data type is money (8 bytes).
- Null Block (aka Null Bitmap) – 3 bytes
The nulls in the money field do not change the space consumption because fixed length data types always use the full amount of space allocated to them. At 21 bytes per row, 1000 rows of the Bonus table would require 21,000 bytes and fill up about 3 data pages (1 data page = 8060 bytes). Currently there are only three records in this table so just 1 data page of 8K has started to fill up.
Analyzing Space Used
A handy tool for checking the storage amount which an object occupies is sp_spaceused. The figure below shows the Bonus table passed into this stored procedure. We see the Bonus table contains three rows and its data has not yet exceeded its first 8 KB page.
If you added enough records the data space used would need more memory pages. Let’s run a loop to add 997 more records to the Bonus table. The first record populated will be row 4. This loop increments each subsequent BonusID value by 1, and the loop runs as long as the BonusID value is <=1000. Once the row containing BonusID 1000 has been entered into the table, the loop will end. One run the Bonus table contains 1000 records.
Since one row occupies 21 bytes, we know these 1000 rows will take up 21 KB of space and should fit within three data pages.
[21000 bytes/(8060 bytes/page) = 2.61 pages]
Since you can use half a data page SQL dedicated 3 data pages to the Bonus table. Let’s rerun the sp_spaceused sproc and confirm the number of data pages.
Using the Sparse Data Option
The sparse data option is a new SQL Server 2008 feature for fields you expect to be predominantly null. Using the sparse data option, you can instruct SQL Server to not have nulls consume space in sparsely populated fields.
To test this we’re going to delete the Bonus table and then re-create it using the same steps we took previously. The only difference will be that the BonusAmount field will be created using the sparse option. Recall we expect the BonusAmount field to contain very little actual data – most records will be null.
Recall we expect the BonusAmount field to contain very little actual data – most records will be null. The Bonus table now contains 1000 records and BonusAmount is a sparse field, but so far we don’t see any difference in the appearance of the table or the data.
Now let’s review how much space the Bonus table is consuming. Let’s confirm the space usage is less in this example. Had you not used the sparse option the consumption would have been 24KB like in our last example.
Note: If you want to setup the sample JProCo database on your system you can watch this video. For this post you will want to run the SQLArchChapter4.0Setup.sql script from Volume 3.
You have two fields, of INT and MONEY, in your Bonus table. You have 1000 records and all instances of the money column are null. When you set up the money field, you used the Sparse option. How much space are the 1000 rows of the money field using?
- 4000 bytes
- 8000 bytes
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