SQL Server 2008: Administering Database Objects - Working with Tables (part 6) - Compression

7. Compression

You can enable compression on tables and indexes in SQL Server 2008 to reduce the disk space needed to store the data in those tables and indexes. Table and index compression is only available in the Enterprise and Developer Editions of SQL Server. You can implement two different levels of compression: row-level compression and page-level compression.

Row-level compression works by reducing the metadata required to store the data itself. Row-level compression also reduces the amount of storage by using variable length storage for fixed length data types, including numeric data types. For example, a column with a data type int may require 1 byte to store the number 1 and 2 bytes to store the number 1,000 instead of requiring 4 bytes of storage for every single value.

Page-level compression works by implementing row-level compression along with two other techniques called prefix compression and dictionary compression. Prefix compression works by identifying a common value for the data in each column that can be used to reduce storage requirements. The value is then replaced by a pointer to the value that is stored in the compression information structure stored immediately after the page header. Dictionary compression also replaces values on the page with a pointer to a common value in the compression information structure. The main difference is that dictionary compression is not limited to a single column; it looks for common values on the entire page. Page compression does not occur immediately, as there would be no benefit in compressing a nearly empty page.

There are a few things to keep in mind when using data compression. Data compression requires less input and output (IO) because it requires fewer disk reads to access the data. However, data compression will increase the CPU because SQL Server now has to compress and uncompress the data. Page-level compression provides a higher compression ratio than row-level compression, but you may see higher levels of CPU utilization when using pagelevel compression. Deciding whether to use row-level, page-level, or no compression at all is entirely dependent on how each option affects your environment. You could see drastic space savings and increased performance by enabling compression; however, if you are already CPU bound, you may hinder performance by enabling compression.

You can use the sp_estimate_data_compression_savings stored procedure to determine the estimated space you will save by enabling row or page compression, as shown in Figure 4.

Figure 4. Estimated savings using page compression on the CustInfo table

To create a compressed table, you need to specify the compression type (row or page) you would like following the table definition. Listing 20 shows the correct syntax to create a table with page compression.

Example 20. Syntax to Enable Page Compression When Creating a Table

USE AdventureWorks2008
GO

--Drop the table if it currently exists
IF OBJECT_ID('dbo.CustInfo', 'U') IS NOT NULL
  DROP TABLE dbo.CustInfo;

--Create the table with page compression
CREATE TABLE Orders
(OrderID int identity,
OrderDate DateTime NOT NULL)
WITH (DATA_COMPRESSION = PAGE)

To add compression or change the existing compression on a table, you can use the ALTER TABLE statement to rebuild the current pages with the selected compression type. Listing 21 changes the compression to row-level compression for the Orders table. To remove compression, all you need to do is change the value to DATA_COMPRESSION = NONE.

Example 21. Syntax to Add or Change the Compression on an Existing Table

ALTER TABLE Orders
REBUILD WITH (DATA_COMPRESSION = ROW)