Indexes in SQL Server Relational Databases

Indexes are special lookup tables that the database search engine can use to speed up data retrieval operations on a table. An index is a data structure (e.g., B-tree) that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the data structure. Choosing the right indexes is crucial for optimizing database performance.

Types of Indexes

Clustered Indexes

A clustered index defines the physical order of data in the table. Because data pages can only be stored in one order, a table can have only one clustered index. The clustered index key is used to physically sort and store the data rows in the table. This makes queries that search for a range of values on the clustered index key columns very efficient.

• The leaf nodes of a clustered index contain the actual data pages of the table.
• Typically created on the primary key, but can be on any column or set of columns.
• Consider the distribution of data and query patterns when choosing a clustered index key.

Nonclustered Indexes

A nonclustered index is a data structure separate from the data rows that contains the nonclustered index key values and a row locator. The row locator points to the data row that contains the matching key value. A table can have multiple nonclustered indexes.

• The leaf nodes of a nonclustered index contain index key values and pointers (row locators) to the data rows.
• For tables with a clustered index, the row locator is the clustered index key.
• For tables without a clustered index (heap tables), the row locator is a Row ID (RID).
• Useful for columns frequently used in WHERE clauses or JOIN conditions that are not part of the clustered index.

Index Operations

Creating Indexes

Use the CREATE INDEX statement to create indexes.

-- Example of creating a nonclustered index
CREATE NONCLUSTERED INDEX IX_CustomerName
ON Customers (LastName, FirstName);

-- Example of creating a clustered index on a primary key
ALTER TABLE Products
ADD CONSTRAINT PK_ProductID PRIMARY KEY CLUSTERED (ProductID);

Maintaining Indexes

Indexes need to be maintained to ensure optimal performance. Over time, as data is inserted, updated, and deleted, indexes can become fragmented.

• Fragmentation: Occurs when the logical order of data in the index does not match the physical order of data on disk.
• Reorganize: A less intensive operation that defragments the index by filling empty pages and ordering pages.
• Rebuild: A more intensive operation that creates a new index structure, removes fragmentation, and updates statistics.

You can use ALTER INDEX REORGANIZE or ALTER INDEX REBUILD for maintenance.

Index Considerations

• Index Selectivity: The ratio of unique values to the total number of rows. Highly selective indexes are more effective.
• Query Patterns: Analyze your common queries to identify columns frequently used in WHERE clauses, JOIN conditions, and ORDER BY clauses.
• Covering Indexes: A nonclustered index that includes all the columns needed to satisfy a query from the index itself, without needing to access the base table.
• Over-indexing: Too many indexes can slow down data modification operations (INSERT, UPDATE, DELETE) and consume excessive storage space.