In SQL Server, an index is a database object that improves the performance of queries executed on a table. It is created on one or more columns of a table, and stores a sorted copy of the data in those columns. This sorted copy makes it faster for the SQL Server to locate specific values within those columns when executing a query.
An index is similar to the index found at the back of a book. The index contains an ordered list of the pages in the book where a particular word or phrase can be found. Similarly, an index on a table in SQL Server contains an ordered list of the rows in the table, along with a pointer to the location of each row on the disk.
When a query is executed that involves a column or columns covered by an index, SQL Server can use the index to locate the relevant rows much faster than if it had to scan the entire table. This can result in significant performance improvements, especially for large tables.
There are different types of indexes that can be created in SQL Server, depending on the needs of the database and the queries that will be executed. The most common types are:
- Clustered index: This is an index that determines the physical order of the data on disk. A table can have only one clustered index, and it is typically created on the primary key column of the table.
- Non-clustered index: This is an index that does not affect the physical order of the data on disk. A table can have multiple non-clustered indexes, and they are typically created on columns frequently used in search conditions, join conditions, or order by clauses.Creating an index requires some overhead, as SQL Server has to maintain the sorted copy of the data and update it whenever the underlying data changes. Therefore, it is important to carefully consider which columns to index and how many indexes to create, in order to achieve the best query performance while minimizing the overhead of index maintenance.
Here is an example of creating a non-clustered index on a table in SQL Server:
CREATE NONCLUSTERED INDEX idx_customer_lastname
ON Customer (LastName);This creates a non-clustered index named "idx_customer_lastname" on the "LastName" column of the "Customer" table. The index will store the sorted copy of the LastName column, along with a pointer to the corresponding row in the table. Queries that involve searching or ordering by LastName are likely to benefit from this index.