SQL Performance: Indexing - Microsoft Learn

Optimizing SQL Server Performance with Indexing

Indexing is a fundamental technique for improving the performance of database queries. By creating indexes on columns that are frequently used in search conditions (WHERE clauses), join operations, and sorting (ORDER BY clauses), you can significantly speed up data retrieval.

What is an Index?

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 engine to quickly locate specific rows without having to scan the entire table. Indexes are typically implemented as B-trees.

Types of Indexes

Clustered Indexes: Defines the physical order of data in a table. A table can have only one clustered index. The leaf nodes of the clustered index contain the actual data rows.
Nonclustered Indexes: A nonclustered index contains key values and pointers to the actual data rows. The leaf nodes of a nonclustered index store pointers to the data pages in the clustered index or heap.
Unique Indexes: Ensures that all values in the indexed column(s) are unique.
Filtered Indexes: An optimized nonclustered index for queries that select from a well-defined subset of rows.
Columnstore Indexes: Designed for data warehousing workloads, providing high compression and parallel processing for analytical queries.

Creating and Managing Indexes

Indexes can be created using the CREATE INDEX statement. It's crucial to choose the right columns to index based on query patterns and data distribution.


-- Example: Creating a nonclustered index on the 'CustomerID' column of an 'Orders' table
CREATE NONCLUSTERED INDEX IX_Orders_CustomerID
ON Orders (CustomerID);

-- Example: Creating a unique index on the 'Email' column of a 'Users' table
CREATE UNIQUE INDEX UQ_Users_Email
ON Users (Email);

Best Practices for Indexing

Index Frequently Queried Columns: Focus on columns used in WHERE, JOIN ON, and ORDER BY clauses.
Consider Composite Indexes: For queries with multiple conditions, a composite index on multiple columns can be beneficial. The order of columns in the index matters.
Avoid Over-Indexing: Too many indexes can slow down data modification operations (INSERT, UPDATE, DELETE) as each index needs to be maintained.
Monitor Index Usage: Regularly check for unused or redundant indexes and remove them. SQL Server Dynamic Management Views (DMVs) can help identify these.
Understand Index Fragmentation: Fragmented indexes can degrade performance. Regularly rebuild or reorganize indexes to combat fragmentation.
Use Covering Indexes: An index that includes all the columns required by a query in its leaf level can avoid table lookups, improving performance.

Tools and Monitoring

SQL Server provides various tools and DMVs to help analyze and manage indexes:

SQL Server Management Studio (SSMS): Offers graphical tools for index analysis and management.
Dynamic Management Views (DMVs): Such as sys.dm_db_index_usage_stats and sys.dm_db_missing_index_details.
Query Store: Helps identify and analyze query performance regressions, often related to indexing.

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