In this article, we will explore three methods that can be used to eliminate the SORT operator when performing a SELECT statement with an ORDER BY clause in SQL Server. These methods can be particularly useful when the SORT operation is the main performance bottleneck of a SELECT query.

For our demonstration, we will be using the Adventure Works database and the Production.Product table. This table has a Clustered Primary Key defined on the ProductID and three other indexes. We will be disabling one of the indexes to avoid interference with the results.

The four tests we will be running are:

1. Sorting the data on retrieval
2. Using an INDEXED VIEW created on the source table
3. Using a CLUSTERED INDEX on the source table
4. Using a NON-CLUSTERED Index

Let’s take a closer look at each of these tests and their results.

Test 1: Sorting the data on retrieval

In this test, we specify an ORDER BY clause in the SELECT statement to sort the data on retrieval. This is the benchmark test against which we will compare the other three tests.

Test 2: Using an INDEXED VIEW

In this test, we create a view on the source table and create a clustered index on the view. The indexed view physically saves the data in a permanent state in the database and is automatically updated when the source tables are updated. We then run our SELECT statement against the indexed view.

Test 3: Using a CLUSTERED INDEX

In this test, we remove the CLUSTERED INDEX definition from the Primary Key and specify it on other columns, specifically the column we want to sort by. We create a replica of the source Product table for this test to avoid modifying the original table.

Test 4: Using a NON-CLUSTERED Index

In this test, we create a NON-CLUSTERED index on the source table, with the sort columns as the index columns and the remaining columns defined as ‘Included columns’.

Comparing the results of these tests, we can see that each method has its own advantages and disadvantages. The choice of method depends on the specific scenario and requirements.

For example, sorting the data on retrieval has the advantage of not requiring any changes to the database structure and not affecting INSERT, UPDATE, and DELETE performance. However, it introduces the SORT operator and can have a larger query cost.

Using an indexed view can result in faster sorting since the data is selected from an already sorted source. However, it may not always be possible to make changes to the database, and DML statements will run slower due to the need to update both the source table and the indexed view.

Using a clustered index on the source table also provides faster sorting, but it may not always be feasible to make changes to the database. If the clustered index column changes frequently, DML statements will be slower due to the continuous updates of the clustered index.

Using a non-clustered index on the source table offers faster sorting as the data is selected from an already sorted source. It does not require changes to the original table design. However, like the other methods, it can slow down DML statements as the SQL engine needs to update both the source table and the extra index.

In conclusion, there is no one-size-fits-all solution for eliminating the SORT operator in SQL Server. The best approach will depend on the specific scenario and the trade-offs between sorting performance and the impact on DML statements. Proper testing is essential to determine the most suitable method for each situation.

Remember, in a transactional system (OLTP), the extra cost of DML statements may not be acceptable, and all methods should be discarded. In a Data Warehouse system, where slowing down DML statements during the ETL process may be acceptable, all methods can be considered.

By understanding these methods and their pros and cons, you can optimize your SQL Server queries and improve performance in your database applications.