generate barcode in vb.net Lesson 3: Planning Indexes in C#

Draw UPCA in C# Lesson 3: Planning Indexes

Lesson 3: Planning Indexes
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Keep in mind that this is a char(60) column. If the index were created on an int column requiring 4 bytes of storage instead of the 60 bytes required for a char(60) column, it would require a single page to be read to locate a page until the 2,016th row was entered. You could add a few more than 4 million rows to the table and need to read only two pages to find a row. It would take more than 8 billion rows in the table before SQL Server would need to read three pages to find the data you were looking for.
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What are the three possible levels of an index And how many pages can they contain An index has a single page at the root level, which is referred to as the root page. An intermediate level can contain zero or more pages, and an index can have multiple intermediate levels. The leaf level can contain one or more pages, and there is only a single leaf level that is at the bottom of the index.
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Clustered Indexes
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The column(s) defined for the clustered index are referred to as the clustering key. A clustered index causes SQL Server to order the data in the table according to the clus tering key. Because a table cannot be sorted more than one way, you can define only one clustered index on a table. Additionally, the leaf level of a clustered index is the actual data within the table. In other words, when the leaf level of a clustered index is reached, SQL Server does not have to use a pointer to access the actual data in the table because it has already reached the actual data pages in the table.
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It is a common misconception that a clustered index causes the data to be physically ordered in a table. A clustered index causes the rows in a table to be ordered according to the clustering key, as well as the data pages in the doubly linked list that stores all the data of a table to also be ordered according to the clustering key. However, this is still a logical ordering. The rows of a table can be stored on the physical disk platters all over the place. If a clustered index could cause a physical ordering (which means the storage location on disk), it would create a prohibitive amount of disk activity.
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12
Designing the Physical Database
As a general rule of thumb, every table should have a clustered index, and this clus tered index should also be the primary key.
IMPORTANT
Clustered index selection
Several readers might be turning purple when they read that the clustered index should also be the primary key, but general rule of thumb does not mean always. The primary key is not always the best choice. However, we didn t have 200+ pages to devote to explaining all the permutations and considerations for selecting the perfect clustering key. Even if we did have the space to devote to the topic, we would still end up with the same general rule of thumb. The Inside SQL Server series from Microsoft Press has all the additional information required to make the necessary choices. Clustering the primary key is always a better choice than not having a clustered index at all.
The general syntax to create a clustered index is as follows:
CREATE [ UNIQUE ] [ CLUSTERED | NONCLUSTERED ] INDEX index_name ON <object> ( column [ ASC | DESC ] [ ,...n ] ) [ INCLUDE ( column_name [ ,...n ] ) ] [ WITH ( <relational_index_option> [ ,...n ] ) ] [ ON { partition_scheme_name ( column_name ) | filegroup_name | default } ][ ; ]
We have already covered the UNIQUE keyword in Lesson 1. All primary keys and unique constraints are implemented as unique indexes. The CLUSTERED and NONCLUSTERED options designate the type of index that is created. Every index must have a name that conforms to the rules for object identifiers. The ON clause specifies the object to create the index against. It can be either a table or a view. (Indexed views will be covered in Lesson 4). The list of columns to create the index on is included in parentheses following the table or view. The ASC and DESC keywords specify whether the sort order should be ascending or descending. The ON clause specifies the physical storage to place the index on. It can be either a filegroup or a partition scheme.
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