SQL Index is a database object that is used to improve the performance of database operations. It is a data structure that is created on one or more columns of a table to allow faster retrieval of data. An index is similar to a book index, which helps you find information quickly by providing a list of page numbers where the information is located.
When you execute a query that involves a large amount of data, the database engine has to scan the entire table to find the required data. This can be a time-consuming process, especially if the table contains millions of rows. However, if an index is created on the columns used in the query, the database engine can use the index to quickly locate the required data, resulting in faster query execution.
Indexes can be created on one or more columns of a table, and they can be of different types, such as clustered, non-clustered, unique, and composite. Each type of index has its own advantages and disadvantages, and the choice of index type depends on the specific requirements of the application.
To create an index in SQL, you can use the CREATE INDEX statement. The syntax of the statement is as follows:
CREATE [UNIQUE] [CLUSTERED | NONCLUSTERED] INDEX index_name ON table_name (column1 [ASC | DESC], column2 [ASC | DESC], ...)
The CREATE INDEX statement creates an index with the specified name on the specified table and columns. The UNIQUE keyword specifies that the index should not allow duplicate values. The CLUSTERED keyword specifies that the index should be a clustered index, which means that the data in the table is physically sorted based on the index key. The NONCLUSTERED keyword specifies that the index should be a non-clustered index, which means that the data in the table is not physically sorted based on the index key.
Here is an example of creating a non-clustered index on the "customer_id" column of the "orders" table:
CREATE INDEX idx_orders_customer_id ON orders (customer_id);
Once an index is created, the database engine can use it to speed up query execution. When you execute a query that involves the indexed columns, the database engine checks if there is an index that can be used to retrieve the required data. If there is an index, the database engine uses it to quickly locate the required data. If there is no index, the database engine has to scan the entire table to find the required data.
Here is an example of using an index in a SELECT statement:
SELECT * FROM orders WHERE customer_id = 123;
If there is an index on the "customer_id" column of the "orders" table, the database engine can use it to quickly locate the rows where the customer_id is 123, resulting in faster query execution.
SQL Index is a powerful tool that can significantly improve the performance of database operations. By creating indexes on the columns used in queries, you can speed up query execution and reduce the time required to retrieve data from large tables. However, it is important to choose the right type of index and to avoid creating too many indexes, as this can have a negative impact on performance.