Indexing in SQL Server

Indexing is a critical database operation that improves the speed of data retrieval operations (SELECT statements) on a database table. Indexes can also be used to enforce uniqueness on rows, such as a primary key constraint.

Types of Indexes

Clustered Indexes

A clustered index determines the physical order of data in the table. Because a table can have only one clustered index, the clustered index key is often the PRIMARY KEY for the table.

• Leaf Nodes: The leaf nodes of a clustered index contain the actual data pages of the table.
• Performance: Excellent for range queries and for retrieving data in sorted order.
• Considerations: Can cause fragmentation on INSERT, UPDATE, and DELETE operations if not carefully managed.

Nonclustered Indexes

A nonclustered index contains the index key values and pointers to the actual data rows. The data rows are stored separately in a heap or clustered index. A table can have multiple nonclustered indexes.

• Leaf Nodes: The leaf nodes of a nonclustered index contain pointers to the data rows. These pointers can be the Row Identifier (RID) if the table is a heap, or the clustered index key if the table has a clustered index.
• Performance: Good for queries that search for specific values or a narrow range of values.
• Covering Indexes: A nonclustered index can be a "covering index" if it includes all the columns required by a specific query, allowing the query to be satisfied solely from the index without accessing the base table.

Other Index Types

• Unique Indexes: Enforces that the index key contains no duplicate values.
• Filtered Indexes: An optimized nonclustered index on a specific subset of rows in a table, defined by a WHERE clause.
• Columnstore Indexes: Designed for data warehousing workloads, storing data in a columnar format for efficient analytical queries.
• Hash Indexes: Available for memory-optimized tables, providing equality lookups.
• Full-Text Indexes: Used for performing linguistic searches on character-based data.

Index Maintenance

Indexes can become fragmented over time, which can degrade query performance. Regular maintenance is essential:

• Reorganize: Defragments the index by merging pages and eliminating empty space.
• Rebuild: Recreates the index from scratch, removing fragmentation and optionally updating statistics.

Best Practices

• Index columns used frequently in WHERE clauses, JOIN conditions, and ORDER BY clauses.
• Avoid over-indexing, as each index adds overhead to data modification operations and consumes storage space.
• Choose the appropriate index type for your workload.
• Regularly review and maintain indexes.
• Consider covering indexes for frequently executed queries.