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- 1. 8.1 : Indexes SQL Server 2005
Hinweis der Redaktion
- Faculty Notes : For example : Indexes in SQL server are like an index that are given at back of any book which helps in finding topics you are looking for.
- Faculty Notes: Maintaining indexes requires time and resources. User should not create an index that would not be used frequently. The clustered index should be created before non clustered index. Clustered index changes the order of rows. The non clustered index would need to be rebuilt if it is built before clustered index. Typically, non clustered indexes are created on foreign keys
- Faculty Notes: How clustered indexes work SQL server 2005 performs the following steps when it uses a clustered index to search for a value: 1. SQL server 2005 obtains the address of the root page from sysindexes table. 2. The search values are compared with the keys of the root page 3. The highest key value is found on the page where the key value is less than or equal to the search value 4. The page pointer is followed to the next lowest level in the index 5. Steps 3 and 4 are repeated until the data page is reached 6. The rows of data are searched on the data page until the search value is found. If search value is not found on the data page ,no rows are returned by the query.
- Faculty notes: SQL server creates non clustered index by default .The data is present in random order but the logical ordering is specified by the index. The data rows may be randomly spread throught the table The non clustered index tree contains the index keys in the sorted order, with the leaf level of the index containing the pointer to the data page and the row no in the data page. SQL server 2005 performs the following steps when it uses a non clustered index to search for value: 1.SQL server obtains the address of the root page from the sysindexes table. 2. The search values are compared with the keys of the root page 3. The Page with the highest key value less than or equal to the search value is found 4. The page pointer is followed to the next lowest level in the index 5. Steps 3 and 4 are repeated until the data page is reached 6.The rows are searched on the leaf page for the specified value .If a match is not found; the table contains no matching rows. 7.If a match is found, the pointer is followed to the data page and row-id in the table, and the requested row is retrieved Non Clustered Index : The data is stored in one place, the index in another, with pointers to the storage location of the data. The items in the index are stored in the order of the index key values, but the information in the table is stored in a different order (which can be dictated by a clustered index). If no clustered index is created on the table, the rows are not guaranteed to be in any particular order.
- Faculty notes: UNIQUE creates an index in which each row should contain a different index value. CLUSTERED specifies a clustered index in which the data is sorted on the index attribute. NON CLUSTERED specifies a non clustered index that specifies only the logical Ordering of data, physically data is not sorted. Index_name specifies name of the index .it should be unique for a table, but may not be unique within a database. Table_name specifies name of the table that contains the attributes on which the index is to be created. Columns specifies name of the columns on which the index would be created.
- Faculty Notes Examples This example removes the index named au_id_ind in the authors table. USE pubs IF EXISTS (SELECT name FROM sysindexes WHERE name = 'au_id_ind') DROP INDEX authors.au_id_ind GO
- Faculty Notes : This Examples shows that user can create the index using create command Create index pk1_index on employees (EmployeeID) pk1_index is the Index name Employees is the table name present in Northwind database To view the index whether index is created or not use the following statements sp_help employees. Sp_helpindex employees To drop the index use the following command drop index employees.pk1_index