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. [2000, 2005, 2008] Updated 2-3-2009

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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. [2000, 2005, 2008] Updated 2-3-2009

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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. [2000, 2005, 2008] Updated 2-3-2009

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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. [2000, 2005, 2008] Updated 2-3-2009

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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. [2000, 2005, 2008] Updated 2-3-2009

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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.  [2000, 2005, 2008] Updated 2-3-2009

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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. [2000, 2005, 2008] Updated 2-3-2009

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Avoid using FLOAT or REAL data types for primary keys, as they add unnecessary overhead that hurts performance. Use one of the integer data types instead. [2000, 2005, 2008] Updated 2-3-2009

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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-sargeble, which means that portion of the query cannot make good use an index. [2000, 2005, 2008] Updated 2-3-2009

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If you are using fixed length columns (CHAR, NCHAR) in your table, consider avoiding 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:

  • NA: Not applicable
  • NYN: Not yet known
  • TUN: Truly unknown

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. [2000, 2005, 2008] Updated 2-3-2009

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