https www free barcode generator net upc a Lesson 2: Optimizing Index Strategies in .NET framework

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Lesson 2: Optimizing Index Strategies
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Table 7-4
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Query Performance Matrix
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Query #1 (1 row) Query #2 (selective) 24,237 pages 1,062 pages 28 pages Query #3 (somewhat selective) 24,237 pages 5,029 pages 139 pages Query #4 (non-selective) 24,237 pages 50,125 pages 1,286 pages
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No index Non-covered index Covered index
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24,237 pages 4 pages 3 pages
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-- Drop the table used in this example. DROP TABLE Test.CoveredIndexTest;
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The performance metric that is shown in the table is the number of data pages that SQL Server touched during the query execution (SET STATISTICS IO ON, logical reads). Note that the so-called selective query (Query #2) returns 0.01 percent of the rows in the table. For a table of this size, that still amounts to 1,000 rows. If you are speaking to someone about the number of rows that will be affected by a query, and he or she says that number is only a couple of percent of the table, this usually translates to a lot of rows. Some conclusions that we can draw from the test are given here. (This is only with regard to read performance; write performance is discussed later in this lesson.)
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A covered index always performs better than a non-covered index. For queries that return a very limited number of rows, a non-covered index also performs very well. For the somewhat-selective query (Query #3), the covered index reads at least 36 times fewer pages than the non-covered index. In this case, a query was considered selective by the query optimizer when it matched less than roughly 0.77 percent of the table.
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Using Included Columns and Index Depth
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In earlier versions of SQL Server (before 2005), creating covered non-clustered indexes could often be impossible because an index could contain no more than 16 columns or be more than 900 bytes wide. The new included column feature makes it possible to add columns to an index without adding them to the index s key. Included columns cannot be used for tasks such as filtering or sorting; their sole benefit is reducing page reads through covering queries, thereby avoiding table lookups. An index can have a maximum of 1,023 included columns (a table can have a maximum of 1,024 columns), making it possible to create a non-clustered index that covers the entire table, which is almost like having a second clustered index! In addition, columns that use one of the
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Optimizing SQL Server 2005 Performance
large data types (VARCHAR(max), NVARCHAR(max), VARBINARY(max), XML, TEXT, NTEXT, and IMAGE) are allowed to be included in an index as an included column. Only columns that are used for filtering, grouping, or sorting should be part of the index key; all other columns included in the index should be included columns. Besides allowing for more columns in the index, included columns have other benefits. In the following SQL script, a table with 1,000,000 rows is created with two indexes. One index has all columns in the index key, while the other index only has one column in the key (the one that would be filtered on), and the rest of the columns are included. The width of each row in the index will be a little over 300 bytes; this might sound like a very wide index row, but it not uncommon. This also makes up for the fact that the test table only contains one million rows; for larger tables, the width of the index does not need to be this big to make a performance difference. The following script defines (and populates) objects and indexes used in the following examples:
CREATE TABLE Test.IncludedColumnsTest( PKCol UNIQUEIDENTIFIER NOT NULL DEFAULT NEWSEQUENTIALID() PRIMARY KEY CLUSTERED ,Col1 INT IDENTITY NOT NULL ,Col2 CHAR(20) NOT NULL ,Col3 CHAR(20) NOT NULL ,Col4 CHAR(20) NOT NULL ,Col5 CHAR(20) NOT NULL ,Col6 CHAR(20) NOT NULL ,Col7 CHAR(20) NOT NULL ,Col8 CHAR(20) NOT NULL ,Col9 CHAR(20) NOT NULL ,Col10 CHAR(20) NOT NULL ,Col11 CHAR(20) NOT NULL ,Col12 CHAR(20) NOT NULL ,Col13 CHAR(20) NOT NULL ,Col14 CHAR(20) NOT NULL ,Col15 CHAR(20) NOT NULL ,Col16 CHAR(20) NOT NULL ); INSERT Test.IncludedColumnsTest (Col2, Col3, Col4, Col5, Col6, Col7, Col8, Col9, Col10, Col11, Col12, Col13, Col14, Col15, Col16) SELECT TOP(1000000) CAST(message_id AS CHAR(20)) AS Col2 ,CAST(message_id AS CHAR(20)) AS Col3 ,CAST(message_id AS CHAR(20)) AS Col4 ,CAST(message_id AS CHAR(20)) AS Col5 ,CAST(message_id AS CHAR(20)) AS Col6 ,CAST(message_id AS CHAR(20)) AS Col7 ,CAST(message_id AS CHAR(20)) AS Col8 ,CAST(message_id AS CHAR(20)) AS Col9 ,CAST(message_id AS CHAR(20)) AS Col10 ,CAST(message_id AS CHAR(20)) AS Col11
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