пятница, 5 ноября 2010 г.

Use varchar(max) instead of text in SQL Server

As you may know on SQL Server a varchar column can hold up to 8000 characters (each row can hold up to 8K, so it depends on the size of other columns), when you need to store more it is common to use the text or ntext datatypes. The problem with the text datatype however is that you can't call most string functions on the column, they also require more IO due to how they are stored internally.
In SQL Server 2005 Microsoft added support for varchar(max) and nvarchar(max), this new datatype can be used anywhere a regular length limited varchar can, but lets you store up to 2GB of data. Behind the scenes the varchar(max) stores up to the as much of the text as it can in the row (up to 8K), and then creates additional pages for any additional text. So in a lot of cases the text content will fit in the row, requiring much less disk IO.
Microsoft is said to be deprecating the text and ntext in future releases.
I also found an interesting blog entry which finds that when you alter a column from ntext/text to nvarchar(max)/varchar(max) the text content will still be stored in the external page, you should run UPDATE tableName SET columnName=columnName which will cause SQL server to store text more efficiently.

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