what is indexes in sqlserver ., advantages of indexes and use

Question

sathish kumar · Answer

Creating index on an existing table is a faster way to retrieve the rows When there are 1000 of records in a table , retrieving information will take a long time. Therefore Indexes are created on columns which are accessed frequently , so that the information can be retrieved quickly. Indexes can be created on a single column or a group of columns. When index is created , it first sorts the data and then it assigns ROWID for each row. Syntax: Create Index Index_Name ON Table_Name (Column1, column2.......); I hope this will be useful for u....

Senthilkumar · Answer

Hi Selvi, Indexes are created on columns in tables or views. The index provides a fast way to look up data based on the values within those columns. For example, if you create an index on the primary key and then search for a row of data based on one of the primary key values, SQL Server first finds that value in the index, and then uses the index to quickly locate the entire row of data. Without the index, a table scan would have to be performed in order to locate the row, which can have a significant effect on performance. Clustered Indexes A clustered index stores the actual data rows at the leaf level of the index. Returning to the example above, that would mean that the entire row of data associated with the primary key value of 123 would be stored in that leaf node. An important characteristic of the clustered index is that the indexed values are sorted in either ascending or descending order. As a result, there can be only one clustered index on a table or view. In addition, data in a table is sorted only if a clustered index has been defined on a table. Nonclustered Indexes Unlike a clustered indexed, the leaf nodes of a nonclustered index contain only the values from the indexed columns and row locators that point to the actual data rows, rather than contain the data rows themselves. This means that the query engine must take an additional step in order to locate the actual data. A row locator's structure depends on whether it points to a clustered table or to a heap. If referencing a clustered table, the row locator points to the clustered index, using the value from the clustered index to navigate to the correct data row. If referencing a heap, the row locator points to the actual data row. Advantages: The database engine can use indexes to boost performance in a number of different queries. Sometimes these performance improvements are dramatic. An important feature of SQL Server 2000 is a component known as the query optimizer. The query optimizer's job is to find the fastest and least resource intensive means of executing incoming queries. An important part of this job is selecting the best index or indexes to perform the task. In the following sections we will examine the types of queries with the best chance of benefiting from an index. http://odetocode.com/articles/70.aspx