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Data Type Performance Tuning Tips for Microsoft SQL Server
Always specify the narrowest columns you can. The narrower the column, the less amount of data SQL Server has to store, and the faster SQL Server is able to read and write data. In addition, if any sorts need to be performed on the column, the narrower the column, the faster the sort will be. [6.5, 7.0, 2000, 2005] Updated 8-21-2005
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If you need to store large strings of data, and they are less than 8,000 characters, use a VARCHAR data type instead of a TEXT data type. TEXT data types have extra overhead that drag down performance. [7.0, 2000, 2005] Updated 8-21-2005
If you have a database running on SQL Server 7.0, 2000, or 2005 that used to run under version 6.5, and because of the limited row size had to split a column into two or more columns because the column width exceed what SQL Server version 6.5 was able to support, consider altering the table so that the multiple columns now fit back into one column again. This of course assumes that your column width is 8,000 characters or less for ASCII data. This will reduce server overhead and boost performance. [6.5, 7.0, 2000, 2005] Updated 8-21-2005
Don't use the NVARCHAR or NCHAR data types unless you need to store 16-bit character (Unicode) data. They take up twice as much space as VARCHAR or CHAR data types, increasing server I/O and wasting unnecessary space in your buffer cache. [7.0, 2000, 2005] Updated 8-21-2005
If the text data in a column varies greatly in length, use a VARCHAR data type instead of a CHAR data type. The amount of space saved by using VARCHAR over CHAR on variable length columns can greatly reduce I/O reads cache memory used to hold data, improving overall SQL Server performance.
Another advantage of using VARCHAR over CHAR columns is that sorts performed on VARCHAR columns are generally faster than on CHAR columns. This is because the entire width of a CHAR column needs to be sorted. [6.5, 7.0, 2000, 2005] Updated 8-21-2005.
If a column's data does not vary widely in length, consider using a fixed-length CHAR field instead of a VARCHAR. While it may take up a little more space to store the data, processing fixed-length columns is faster in SQL Server than processing variable-length columns. [6.5, 7.0, 2000, 2005] Updated 8-21-2005
Always choose the smallest data type you need to hold the data you need to store in a column. For example, if all you are going to be storing in a column are the numbers 1 through 10, then the TINYINT data type is more appropriate that the INT data type. The same goes for CHAR and VARCHAR data types. Don’t specify more characters in character columns that you need. This allows you to store more rows in your data and index pages, reducing the amount of I/O needed to read them. It also reduces the amount of data moved from the server to the client, reducing network traffic and latency. And last of all, it reduces the amount of wasted space in your buffer cache. [6.5, 7.0, 2000, 2005] Updated 8-21-2005
If you have a column that is designed to hold only numbers, use a numeric data type, such as INTEGER, instead of a VARCHAR or CHAR data type. Numeric data types generally require less space to hold the same numeric value as does a character data type. This helps to reduce the size of the columns, and can boost performance when the columns is searched (WHERE clause), joined to another column, or sorted. [6.5, 7.0, 2000, 2005] Updated 10-16-2005
Don't use FLOAT or REAL data types for primary keys, as they add unnecessary overhead that hurts performance. Use one of the integer data types instead. [6.5, 7.0, 2000, 2005] Updated 5-15-2006
When specifying data types during table creation, always specify NULL or NOT NULL for each column. If you don't, then the column will default to NOT NULL if the ANSI NULL DEFAULT database option is not selected (the default), and will default to NULL of the ANSI NULL DEFAULT database option is selected.
For best performance, and to reduce potential code bugs, columns should ideally be set to NOT NULL. For example, use of the IS NULL keywords in the WHERE clause makes that portion of the query non-sargable, which means that portion of the query cannot use an index. [6.5, 7.0, 2000, 2005] Updated 5-15-2006
If you are using fixed length columns (CHAR, NCHAR) in your table, do your best to avoid storing NULLs in them. If you do, the entire amount of space dedicated to the column will be used up. For example, if you have a fixed length column of 255 characters, and if you place a NULL in it, then 255 characters have to be stored in the database. This is a large waste of space that will cause SQL Server to have to perform extra disk I/O to read data pages. It also wastes space in the data cache buffer. Both of these contribute to reduced SQL Server performance.
Instead of using NULLs, use a coding scheme similar to this in your databases:
Such a scheme provides the benefits of using NULLs, but without the drawbacks.
If you really must use NULLs, use a variable length column instead of a fixed length column. Variable length columns only use a very small amount of space to store a NULL. [7.0, 2000, 2005] Updated 5-15-2006
If you use the CONVERT function to convert a value to a variable length datatype, such as VARCHAR, always specify the length of the variable datatype. If you do not, SQL Server assumes a default length of 30. Ideally, you should specify the shortest length to accomplish the required task. This helps to reduce memory use and SQL Server resources. [6.5, 7.0, 2000, 2005] Updated 5-15-2006
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