What are Indexes in Sql Server

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.

You can create indexes on most columns in a table or a view. The exceptions are primarily those columns configured with large object (LOB) data types, such as image, text, and varchar(max). You can also create indexes on XML columns, but those indexes are slightly different from the basic index

An index is made up of a set of pages (index nodes) that are organized in a B-tree structure. This structure is hierarchical in nature, with the root node at the top of the hierarchy and the leaf nodes at the bottom.

When a query is issued against an indexed column, the query engine starts at the root node and navigates down through the intermediate nodes, with each layer of the intermediate level more granular than the one above. The query engine continues down through the index nodes until it reaches the leaf node.

The leaf node will contain either the entire row of data or a pointer to that row, depending on whether the index is clustered or nonclustered.

Clustered Indexes

A clustered index stores the actual data rows at the leaf level of the index.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.

Note: A table that has a clustered index is referred to as a clustered table. A table that has no clustered index is referred to as a heap.

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.

Nonclustered indexes cannot be sorted like clustered indexes; however, you can create more than one nonclustered index per table or view. SQL Server 2005 supports up to 249 nonclustered indexes, and SQL Server 2008 support up to 999.

Index Types

In addition to an index being clustered or nonclustered, it can be configured in other ways:

• Composite index: An index that contains more than one column. In both SQL Server 2005 and 2008, you can include up to 16 columns in an index, as long as the index doesn’t exceed the 900-byte limit. Both clustered and nonclustered indexes can be composite indexes.
• Unique Index: An index that ensures the uniqueness of each value in the indexed column. If the index is a composite, the uniqueness is enforced across the columns as a whole, not on the individual columns. For example, if you were to create an index on the FirstName and LastName columns in a table, the names together must be unique, but the individual names can be duplicated.

A unique index is automatically created when you define a primary key or unique constraint:

• Primary key: When you define a primary key constraint on one or more columns, SQL Server automatically creates a unique, clustered index if a clustered index does not already exist on the table or view. However, you can override the default behavior and define a unique, nonclustered index on the primary key.
• Unique: When you define a unique constraint, SQL Server automatically creates a unique, nonclustered index. You can specify that a unique clustered index be created if a clustered index does not already exist on the table.

• Covering index: A type of index that includes all the columns that are needed to process a particular query. For example, your query might retrieve the FirstName and LastName columns from a table, based on a value in the ContactID column. You can create a covering index that includes all three columns.