Creating SQL Indexes

What are Indexes?

In a relational database, an index is a data structure that improves the speed of data retrieval operations on a database table. It works much like an index in a book, allowing the database to quickly locate specific rows without scanning the entire table. When you create an index on one or more columns of a table, the database engine can use this index to find the desired data more efficiently.

Indexes are crucial for optimizing query performance, especially in large databases where full table scans can be prohibitively slow.

Why Use Indexes?

• Faster Query Performance: The primary benefit is significantly speeding up `SELECT` queries, especially those with `WHERE` clauses, `JOIN` conditions, and `ORDER BY` clauses.
• Enforcing Uniqueness: Unique indexes can be used to ensure that the values in an indexed column (or combination of columns) are unique across all rows in the table.
• Improved Sorting: Indexes can help in sorting data more quickly if the `ORDER BY` clause matches the indexed columns.

Types of Indexes

SQL Server supports several types of indexes, with the most common being:

• Clustered Indexes: Determines the physical order of data in the table. A table can have only one clustered index. By default, the primary key constraint creates a clustered index.
• Nonclustered Indexes: A separate structure from the data rows, containing pointers to the data rows. A table can have multiple nonclustered indexes.

Other index types include Unique Indexes, Filtered Indexes, Columnstore Indexes, and Full-Text Indexes, each serving specific purposes.

For more details, refer to the Index Types documentation.

Creating a Nonclustered Index

Nonclustered indexes are created using the CREATE NONCLUSTERED INDEX statement. They store the indexed column values and a pointer (row locator) to the actual data row.

Syntax:

CREATE NONCLUSTERED INDEX index_name
ON table_name (column1 [ASC|DESC], column2 [ASC|DESC], ...);
            

Example: To create a nonclustered index on the LastName column of a Customers table:

CREATE NONCLUSTERED INDEX IX_Customers_LastName
ON dbo.Customers (LastName);
            

This index can speed up queries that filter or sort by LastName.

You can also create composite indexes (on multiple columns) and include additional columns using the INCLUDE clause for covering queries.

CREATE NONCLUSTERED INDEX IX_Orders_CustomerID_OrderDate
ON dbo.Orders (CustomerID, OrderDate)
INCLUDE (OrderTotal);
            

Creating a Clustered Index

A clustered index defines the physical storage order of the table's data. The leaf nodes of a clustered index contain the actual data pages.

Syntax:

CREATE CLUSTERED INDEX index_name
ON table_name (column1 [ASC|DESC], column2 [ASC|DESC], ...);
            

Example: To create a clustered index on the CustomerID column of a Customers table (assuming no primary key constraint has already created one):

CREATE CLUSTERED INDEX CX_Customers_CustomerID
ON dbo.Customers (CustomerID);
            

Key Considerations

• Choose columns wisely: Index columns that are frequently used in `WHERE` clauses, `JOIN` conditions, and `ORDER BY` clauses.
• Avoid over-indexing: Too many indexes can slow down write operations and consume disk space.
• Index selectivity: Indexes on columns with high selectivity (many distinct values) are generally more effective than those on columns with low selectivity (few distinct values).
• Maintenance: Regularly monitor index fragmentation and rebuild or reorganize indexes as needed.
• Covering Indexes: Use the `INCLUDE` clause in nonclustered indexes to create covering indexes that can satisfy queries without accessing the base table.