Non-clustered index and table variables in SQL Server

22 Sep 2023

In SQL Server, an index is a data structure that improves the speed of data retrieval operations on a table. It allows SQL Server to quickly locate and retrieve the requested data. While a clustered index determines the physical order of data in a table, a non-clustered index provides an additional way to look up data based on specific columns.

How does a non-clustered index work?

A non-clustered index creates a separate structure that includes the indexed columns and a pointer to the corresponding data row in the table. This structure is stored separately from the table, which allows for faster searching and retrieval of data. When you query a table using a non-clustered index, SQL Server looks up the indexed values, finds the corresponding pointers, and then retrieves the actual data row.

Benefits of using non-clustered indexes

Improved query performance: Non-clustered indexes can significantly speed up data retrieval when querying a table based on non-key columns. They allow SQL Server to quickly locate the desired data without scanning the entire table.
Reduced I/O operations: By using a non-clustered index, SQL Server can minimize the number of disk I/O operations required to retrieve data, resulting in faster query execution.
Efficient sorting and joining: Non-clustered indexes can also be used to optimize sorting and joining operations, as they provide an ordered representation of the indexed columns.

Creating a non-clustered index

To create a non-clustered index on a table, you can use the CREATE INDEX statement in SQL Server. Here’s an example:

CREATE NONCLUSTERED INDEX IX_Employee_LastName
ON Employees (LastName)

In this example, a non-clustered index named IX_Employee_LastName is created on the LastName column of the Employees table.

Table Variables in SQL Server

In SQL Server, a table variable is a variable that can hold a resultset, similar to a temporary table. However, unlike temporary tables, table variables are stored in memory and only exist for the duration of the current batch or procedure.

Benefits of using table variables

Faster execution: Table variables are stored in memory, which can lead to faster execution compared to temporary tables stored on disk.
Reduced contention: Since table variables are scoped to the current batch or procedure, they have limited visibility for other concurrent processes, reducing potential contention issues.
No logging: Table variables are not logged to transaction logs, making them a suitable choice for scenarios where logging overhead needs to be minimized.

Declaring and using table variables

To declare a table variable in SQL Server, you can use the DECLARE statement followed by the table variable name and its columns. Here’s an example:

DECLARE @Employees TABLE
(
    EmployeeID INT,
    FirstName VARCHAR(50),
    LastName VARCHAR(50)
)

In this example, a table variable named @Employees is declared with three columns: EmployeeID, FirstName, and LastName.

Table variables can be used just like any other table in SQL Server. You can insert data into a table variable, perform SELECT, UPDATE, DELETE, or JOIN operations on it.

Conclusion

Non-clustered indexes and table variables are powerful features in SQL Server that can enhance query performance and improve memory usage. Understanding when and how to use them effectively can greatly benefit your SQL Server applications.

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