qr code generator c# mvc Using Included Columns and Reducing Index Depth in C#.NET

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Using Included Columns and Reducing Index Depth
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In versions of SQL Server prior to SQL Server 2005, creating covered nonclustered 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
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Techniques to Improve Query Performance
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used for tasks such as filtering or sorting; their sole benefit is reducing page reads through covering queries by avoiding table lookups. An index can have a maximum of 1,023 included columns, and a table can have a maximum of 1,024 columns, making it possible to create a nonclustered index that covers the entire table, which is almost like having a second clustered index! In addition, columns that use one of the large data types [VARCHAR(max), NVAR-CHAR(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 has only 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 is a little over 300 bytes. This might sound like a very wide index row, but having this kind of width is not uncommon. This also makes up for the fact that the test table contains only 1 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 subsequent examples:
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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
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Lesson 2: Creating Indexes
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,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 ,CAST(message_id AS CHAR(20)) AS Col12 ,CAST(message_id AS CHAR(20)) AS Col13 ,CAST(message_id AS CHAR(20)) AS Col14 ,CAST(message_id AS CHAR(20)) AS Col15 ,CAST(message_id AS CHAR(20)) AS Col16 FROM sys.messages AS sm CROSS JOIN ( SELECT TOP(15) 1 AS Col FROM sys.messages ) AS x;
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CREATE NONCLUSTERED INDEX IncludedColumns ON Test.IncludedColumnsTest (Col1) INCLUDE (Col2, Col3, Col4, Col5, Col6, Col7, Col8, Col9, Col10, Col11, Col12, Col13, Col14, Col15, Col16);
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CREATE NONCLUSTERED INDEX NoIncludedColumns ON Test.IncludedColumnsTest (Col1, Col2, Col3, Col4, Col5, Col6, Col7, Col8, Col9, Col10, Col11, Col12, Col13, Col14, Col15, Col16);
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Table 6-5 shows some of the interesting differences between indexes with and without included columns.
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tabLe 6-5 Index Size Matrix
INCLUDEDCOLUMN
NOINCLUDEDCOLUMN
Total size Size of the nonleaf level of the index Index depth
40,147 pages 146 pages Three levels (a root page + one intermediate level + one leaf level) 27 bytes
41,743 pages 1,743 pages Five levels (a root page + three intermediate levels + one leaf level) 327 bytes
Average size of rows in the nonleaf levels of the index Average size of rows in the leaf level of the index
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