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SQL Server indexing is a highly efficient performance-tuning method designed specifically to speed up the data retrieval operations in SQL Server databases. An index works much like the index of a book, providing quick access to the data saved in tables. By creating appropriate indexes, you can dramatically speed up your query performance.
What is SQL Server Indexing?
An index in SQL Server is a database object that can be linked to a table to speed up the retrieval of records, ensuring that the database engine can find the data quickly. Understandably, indexes are crucial for performance. Just imagine searching for a specific topic in an enormous book without an index. You’d likely have to scan the entire text, right? In database terms, this exhaustive search is known as a table scan. With an extensive dataset, table scans are incredibly inefficient and time-consuming. But with an index in place, SQL Server can swiftly locate the data you need.
Types of Indexes in SQL Server
SQL Server supports numerous types of indexes including Clustered, Nonclustered, Columnstore, and more.
1. Clustered Indexes
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CREATE CLUSTERED INDEX index_name ON table_name (column1, column2, column3, ...) |
A clustered index determines the physical order of data in a table, which means there can be only one clustered index on a table. The leaf nodes of a clustered index contain the data pages.
2. Nonclustered Indexes
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CREATE NONCLUSTERED INDEX index_name ON table_name (column1, column2, column3, ...) |
A nonclustered index doesn’t sort the physical data inside the table; it only sorts the logical order of the data. The leaf node of a nonclustered index doesn’t consist of the data pages, it contains index pages. A table can have multiple nonclustered indexes.
Improving Query Performance with Indexing
The right indexes can make your queries run far faster by limiting the amount of data that must be read. Let’s look at an example.
Without an Index
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SELECT * FROM Employees WHERE EmployeeId = 12345 |
Without an index, the SQL Server engine would perform a table scan and go through every record, row by row, to find the matching records. This consumes a significant amount of resources.
With an Index
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CREATE NONCLUSTERED INDEX idx_Employees_EmployeeId ON Employees (EmployeeId) SELECT * FROM Employees WHERE EmployeeId = 12345 |
With an index on the EmployeeId column, the SQL Server engine can quickly find the desired record(s), thereby improving query performance.
While indexing significantly boosts SQL Server’s performance, it should be used wisely and maintained regularly. Over-indexing can lead to increased storage and negatively impact the performance of data modification operations like insert, update and delete. You need to select only the necessary columns for indexing and periodically check for fragmented indexes, which could degrade performance.
Conclusion
Understand the needs of your database and your queries to create effective indexes. When done correctly, indexing can dramatically improve the performance of your database operations.