Microsoft SQL Server 2008 Internals in Visual Basic .NET

Generation UCC - 12 in Visual Basic .NET Microsoft SQL Server 2008 Internals

Microsoft SQL Server 2008 Internals
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times that are 100 or 1,000 times more than for a properly written query. Thus, if you opt to use a SQL collation, you need to watch that you don t mix varchar and nvarchar casually. Another case where the collation can make a huge difference is when SQL Server has to look at almost all characters in the strings. For instance, look at the following:
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SELECT COUNT(*) FROM tbl WHERE longcol LIKE '%abc%';
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This may execute 10 times faster or more with a binary collation than a nonbinary Windows collation. And with varchar data, this executes up to seven or eight times faster with a SQL collation than with a Windows collation. If you have a varchar column, you can speed this up by forcing the collation as follows:
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SELECT COUNT(*) FROM tbl WHERE longcol COLLATE SQL_Latin1_General_CP_CI_AS LIKE '%abc%';
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If your column is nvarchar, you have to force a binary collation instead, but that would only be possible if users can accept a case-sensitive search. The same considerations apply to the functions CHARINDEX and PATINDEX.
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Special Data Types
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I ll end this section on data types by showing you a few additional data types that you might nd useful. Binary Data Types These data types are binary and varbinary. They are used to store strings of bits, and the values are entered and displayed using their hexadecimal (hex) representation, which is indicated by a pre x of 0x. So a hex value of 0x270F corresponds to a decimal value of 9,999 and a bit string of 0010011100001111. In hex, each two displayed characters represent a byte, so the value of 0x270F represents 2 bytes. You need to decide whether you want your data to be xed or variable length, and you can use some of the same considerations discussed previously for deciding between char and varchar to make your decision. The maximum length of binary or varbinary data is 8,000 bytes. bit Data Type The bit data type can store a 0 or a 1 and can consume only a single bit of storage space. However, if there is only one bit column in a table, it will take up a whole byte. Up to eight-bit columns are stored in a single byte. LOB Data Types SQL Server 2008 allows you to de ne columns with the MAX attribute: varchar(MAX), nvarchar(MAX), and varbinary(MAX). If the number of bytes actually inserted into these columns exceeds the maximum of 8,000, these columns are stored using a special storage format for LOB data. The special storage format is the same one as used for the data types text, ntext, and image, but because those types will be discontinued in a future version of SQL Server, it is recommend that you use the variable-length data types with the MAX speci er for all new development. The varchar(MAX) (or text) data type can store up to 2^31 1 non-Unicode
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characters, nvarchar(MAX) (or ntext) can store up to 2^30 1 (half as many) Unicode characters, and varbinary(MAX) (or image) can store up to 2^31 1 bytes of binary data. In addition, varbinary(MAX) data can be stored as lestream data. We ll cover lestream data in more detail in 7, as well as look at the storage structures for LOB data. cursor Data Type The cursor data type can hold a reference to a cursor. Although you can t declare a column in a table to be of type cursor, this data type can be used for output parameters and local variables. I ve included the cursor data type in this list for completeness, but I won t be talking more about it. rowversion Data Type The rowversion data type is a synonym for what was formerly called a timestamp. When using the timestamp data type name, many people might assume that the data has something to do with dates or times, but it doesn t. A column of type rowversion holds an internal sequence number that SQL Server automatically updates every time the row is modi ed. The value of any rowversion column is actually unique within an entire database, and a table can have only one column of type rowversion. Any operation that modi es any rowversion column in the database generates the next sequential value. The actual value stored in a rowversion column is seldom important by itself. The column is used to detect whether a row has been modi ed since the last time it was accessed by determining whether the rowversion value has changed. sql_variant Data Type The sql_variant data type allows a column to hold values of any data type except text, ntext, image, XML, user-de ned data types, variable-length data types with the MAX speci er, or rowversion (timestamp). I ll describe the internal storage of sql_variant data later in this chapter. Spatial Data Type SQL Server 2008 provides two data types for storing spatial data. The geometry data type supports planar, or Euclidean ( at-earth), data. The geometry data type conforms to the Open Geospatial Consortium (OGC) Simple Features for SQL Speci cation version 1.1.0. The geography data type stores ellipsoidal (round-earth) data, such as Global Positioning Satellite (GPS) latitude and longitude coordinates. These data types have their own methods for accessing and manipulating the data, as well as their own special extended index structures, which are different than the normal SQL Server indexes. Any further discussion of the access methods and storage of spatial data is beyond the scope of this book. table Data Type The table data type can be used to store the result of a function and can be used as the data type of local variables. Columns in tables cannot be of type table. xml Data Type The xml data type lets you store XML documents and fragments in a SQL Server database. You can use the xml data type as a column type when you create a table, or as the data type for variables, parameters, and the return value of a function. XML data has its own methods for retrieval and manipulation. I will not be covering details of working with xml data in this book.
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