As a SQL Server developer, you may have come across cursors in your code. Cursors are a powerful tool that allow you to iterate over a result set and perform operations on each row. However, they come with a significant performance overhead and should be avoided whenever possible.

In this article, we will discuss why you should avoid using cursors in SQL Server and provide an alternative approach using traditional loops.

The Performance Impact of Cursors

Let’s consider a simple example of a cursor that performs a DBCC CHECKDB on each database in the system. While this example may not demonstrate the full impact of cursors, it serves as a proof of concept.

DECLARE @query VARCHAR(100), @dbname SYSNAME
DECLARE BadCursor CURSOR FOR
  SELECT name FROM sys.databases
  WHERE name NOT IN ('master', 'model', 'msdb', 'tempdb')
OPEN BadCursor
FETCH NEXT FROM BadCursor INTO @dbname
WHILE @@FETCH_STATUS = 0
BEGIN
  SELECT @query = 'DBCC CHECKDB(' + QUOTENAME(@dbname) + ')'
  EXEC(@query)
  FETCH NEXT FROM BadCursor INTO @dbname
END
CLOSE BadCursor
DEALLOCATE BadCursor

When comparing the execution plans of the cursor-based query and a more conventional loop, we can see a significant difference. The cursor-based query took 69% of the execution time, while the loop only took 31%. This means that the cursor took at least twice as long as the regular loop.

The inefficiency of cursors becomes even more apparent when scaling to tables with millions of rows. The overhead of cursor allocation and deallocation, combined with the inefficient way the query is executed, can severely impact performance.

An Alternative Approach

To avoid the performance issues associated with cursors, we can use traditional loops instead. In the previous example, we can replace the cursor with a table variable and a while loop:

DECLARE @query VARCHAR(100), @dbname SYSNAME
DECLARE @dblist TABLE (dbname SYSNAME)
INSERT INTO @dblist(dbname)
SELECT name FROM sys.databases
WHERE name NOT IN ('master', 'model', 'msdb', 'tempdb')
WHILE (SELECT COUNT(*) FROM @dblist) > 0
BEGIN
  SELECT TOP 1 @dbname = dbname FROM @dblist
  SELECT @query = 'DBCC CHECKDB(' + QUOTENAME(@dbname) + ')'
  EXEC(@query)
  DELETE FROM @dblist WHERE dbname = @dbname
END

By using a table variable and a while loop, we eliminate the overhead of cursor allocation and deallocation. This approach can significantly improve the performance of our code, especially when dealing with large result sets.

Conclusion

While cursors may seem attractive with their fetch next feature, it is important to consider the performance implications. In most cases, there is a better alternative to using cursors, such as traditional loops.

By avoiding cursors and adopting more efficient coding practices, you can improve the performance of your SQL Server queries and reduce the overall execution time.

Remember, as SQL Server developers, it is our responsibility to write efficient and optimized code. So, the next time you find yourself reaching for a cursor, consider the alternatives and choose the approach that will yield the best performance.

Happy coding!