Performance Tuning SQL Server Cursors

If possible, avoid using SQL Server cursors. They generally use a lot of SQL Server resources and reduce the performance and scalability of your applications. If you need to perform row-by-row operations, try to find another method to perform the task.

Here are some alternatives to using a cursor:
Use WHILE LOOPS
Use temp tables
Use derived tables
Use correlated sub-queries
Use the CASE statement
Perform multiple queries

More often than not, there are non-cursor techniques that can be used to perform the same tasks as a SQL Server cursor.

If you do find you must use a cursor, try to reduce the number of records to process.

One way to do this is to move the records that need to be processed into a temp table first, then create the cursor to use the records in the temp table, not from the original table. This of course assumes that the subset of records to be inserted into the temp table are substantially less than those in the original table.

The lower the number of records to process, the faster the cursor will finish.

If the number of rows you need to return from a query is small, and you need to perform row-by-row operations on them, don’t use a server-side cursor. Instead, consider returning the entire rowset to the client and have the client perform the necessary action on each row, then return any updated rows to the server.

If you have no choice but to use a server-side cursor in your application, try to use a FORWARD-ONLY or FAST-FORWARD, READ-ONLY cursor. When working with unidirectional, read-only data, use the FAST_FORWARD option instead of the FORWARD_ONLY option, as it has some internal performance optimizations to speed performance. This type of cursor produces the least amount of overhead on SQL Server.

If you are unable to use a fast-forward cursor, then try the following cursors, in this order, until you find one that meets your needs. They are listed in the order of their performance characteristics, from fastest to slowest: dynamic, static, and keyset.

Avoid using static/insensitive and keyset cursors, unless you have no other choice. This is because they cause a temporary table to be created in TEMPDB, which increases overhead and can cause resource contention issues.

If you have no choice but to use cursors in your application, try to locate the SQL Server tempdb database on its own physical device for best performance. This is because cursors use the tempdb for temporary storage of cursor data. The faster your disk array, the faster your cursor will be.

Using cursors can reduce concurrency and lead to unnecessary locking and blocking. To help avoid this, use the READ_ONLY cursor option if applicable, or if you need to perform updates, try to use the OPTIMISTIC cursor option to reduce locking. Try to avoid the SCROLL_LOCKS cursor option, which reduces concurrency.

When you are done using a cursor, don’t just CLOSE it, you must also DEALLOCATE it. Deallocation is required to free up the SQL Server resources used by the cursor. If you only CLOSE the cursor, locks are freed, but SQL Server resources are not. If you don’t DEALLOCATE your cursors, the resources used by the cursor will stay allocated, degrading the performance of your server until they are released.

If it is appropriate for your application, try to load the cursor as soon as possible by moving to the last row of the result set. This releases the share locks created when the cursor was built, freeing up SQL Server resources.

If you have to use a cursor because your application needs to manually scroll through records and update them, try to avoid client-side cursors, unless the number of rows is small or the data is static. If the number of rows is large, or the data is not static, consider using a server-side keyset cursor instead of a client-side cursor. Performance is usually boosted because of a reduction in network traffic between the client and the server. For optimum performance, you may have to try both types of cursors under realistic loads to determine which is best for your particular environment.

When using a server-side cursor, always try to fetch as small a result set as possible. This includes fetching only those rows and columns the client needs immediately. The smaller the cursor, no matter what type of server-side cursor it is, the fewer resources it will use, and performance will benefit.

If you need to perform a JOIN as part of your cursor, keyset and static cursors are generally faster than dynamic cursors, and should be used when possible.

If a transaction you have created contains a cursor (try to avoid this if at all possible), ensure that the number of rows being modified by the cursor is small. This is because the modified rows may be locked until the transaction completes or aborts. The greater the number of rows being modified, the greater the locks, and the higher the likelihood of lock contention on the server, hurting performance.

In earlier versions of SQL Server, Transact-SQL cursors were only global to the connection. But in SQL Server 7 and 2000, there are two options to define the scope of a cursor. LOCAL and GLOBAL keywords in the DECLARE CURSOR statement are used to specify the scope of a cursor. A GLOBAL cursor can be referenced in any stored procedure or batch executed by a connection. LOCAL cursors are more secure as they cannot be referenced outside the procedure or trigger unless they are passed back to the calling procedure or trigger, or by using an output parameter. GLOBAL cursors must be explicitly deallocated or they will be available until the connection is closed. For optimum performance, you should always explicitly deallocate a cursor when you are done using it. LOCAL cursors are implicitly deallocated when the stored procedure, the trigger, or the batch in which they were created terminates. We can use LOCAL cursors for more security and better scope of the cursor in our application, which also helps to reduce resources on the server, and boosting performance.

Consider using asynchronous cursors if you expect your result set to be very large. This allows you to continue processing while the cursor is still being populated. While it may not actually speed up your application, it should give the appearance to your end users that something is happening sooner that if they have to wait until the entire cursor is populated.

If you have to use a cursor, break out of the cursor loop as soon as you can. If you find that a problem has occurred, or processing has ended before the full cursor has been processed, then exit immediately.

If you are using the same cursor more than once in a batch of work, (or within more than one stored procedure), then define the cursor as a global cursor by using the GLOBAL keyword. By not closing or deallocating the cursor until the whole process is finished, a fair amount of time will be saved, as the cursor and the data contained will already be defined, ready for you to use.

Sometimes, it is handy to be able to perform some calculation on one or more columns of a record, and then take the result of that calculation and then add it to similar calculations performed on other related records to find a grand total.

For example, let’s say you want to find the total dollar cost of an invoice. An invoice will generally involve a header record and one or more detail records. Each detail record will represent a line item on the invoice. In order to calculate the total dollar cost of an invoice, based on two or more line items, you would need to multiply the quantity of each item sold times the price of each item. Then, you would need to add the total price of each line item together in order to get the total dollar cost of the entire invoice. To keep this example simple, let’s ignore things like discounts, taxes, shipping, etc.

One way to accomplish this task would be to use a cursor like the one we see below (we are using the Northwind database for this example code):

DECLARE @LineTotal money –Declare variables
DECLARE @InvoiceTotal money
SET @LineTotal = 0 –Set variables to 0
SET @InvoiceTotal = 0

DECLARE Line_Item_Cursor CURSOR FOR –Declare the cursor

SELECT UnitPrice*Quantity –Multiply unit price times quantity ordered
FROM [order details]
WHERE orderid = 10248 –We are only concerned with invoice 10248

OPEN Line_Item_Cursor –Open the cursor
FETCH NEXT FROM Line_Item_Cursor INTO @LineTotal –Fetch next record
WHILE @@FETCH_STATUS = 0

BEGIN
SET @InvoiceTotal = @InvoiceTotal + @LineTotal –Summarize line items
FETCH NEXT FROM Line_Item_Cursor INTO @LineTotal
END

CLOSE Line_Item_Cursor –Close cursor
DEALLOCATE Line_Item_Cursor –Deallocate cursor
SELECT @InvoiceTotal InvoiceTotal –Display total value of invoice

The result for invoice number 10248 is $440.00.

What the cursor does is to select all of the line items for invoice number 10248, then multiply the quantity ordered times the price to get a line item total, and then it takes each of the line item totals for each record and then adds them all up in order to calculate the total dollar amount for the invoice.

This all works well, but the code is long and hard to read, and performance is not great because a cursor is used. Ideally, for best performance, we need to find another way to accomplish the same goal as above, but without using a cursor.

Instead of using a cursor, let’s rewrite the above code using set-based Transact-SQL instead of a cursor. Here’s what the code looks like:

DECLARE @InvoiceTotal money
SELECT @InvoiceTotal = sum(UnitPrice*Quantity)
FROM [order details]
WHERE orderid = 10248
SELECT @InvoiceTotal InvoiceTotal

The result for invoice number 10248 is $440.00.

Right away, it is obvious that this is a lot less code and that is it more readable. What may not be obvious is that it uses less server resources and performs faster. In our example — with few rows — the time difference is very small, but if many rows are involved, the time difference between the techniques can be substantial.

The secret here is to use the Transact-SQL “sum” function to summarize the line item totals for you, instead of relying on a cursor. You can use this same technique to help reduce your dependency on using resource-hogging cursors in much of your Transact-SQL code.