T Sql Sybase Cursors

How to `Avoid Using Cursors` in Sybase (T-SQL)

Cursors in SQL can often be a source of frustration, especially when updating legacy code. They can lead to complex and inefficient queries, slowing down database performance. In this blog post, we’ll explore how you can effectively eliminate the use of cursors in your Sybase (T-SQL) queries, making your stored procedures more efficient and easier to maintain.

The Problem with Cursors

Imagine you’re tasked with updating some legacy Sybase code and you stumble upon a cursor. This cursor is used to process a result set, but it’s not very efficient—especially with large datasets. For instance, let’s say you have a result set with around 500,000 rows and 100 distinct values that need to be processed. Using a cursor for such a task can lead to performance issues and more complicated code than necessary.

A Typical Cursor Example

To illustrate, here’s a sample code snippet that uses a cursor to update a temporary table based on a stored procedure:

declare c_lookup_codes for 
select distinct lookup_code 
from #workinprogress

while(1=1) 
begin 
    fetch c_lookup_codes into @lookup_code 

    if @@sqlstatus<>0 
    begin 
        break 
    end 

    exec proc_code_xref @lookup_code @xref_code OUTPUT 

    update #workinprogress 
    set xref = @xref_code 
    where lookup_code = @lookup_code 
end

While this code may work, it’s far from optimal. Let’s explore how we can remove the cursor entirely.

The Solution: Using a XRef Table

To avoid using a cursor, one effective approach is to create a cross-reference (XRef) table. By doing this, you can pre-populate the XRef table with all the necessary values that you need from the stored procedure.

Steps to Eliminate the Cursor

Create the XRef Table: Since you know the distinct lookup values are static, create an XRef table to hold these values.
Insert Values: You can call proc_code_xref 100 times, inserting the results directly into your newly created XRef table.
Perform Updates Using Joins: Instead of fetching values in a loop, you can use a single update statement with a join operation to update your temporary table. This greatly reduces the complexity and enhances performance.

Example Code without Cursors

Here’s how your code might look after implementing these suggestions:

-- Create the XRef table
CREATE TABLE XRef (lookup_code VARCHAR(50), xref_code VARCHAR(50))

-- Populate the XRef table
INSERT INTO XRef (lookup_code, xref_code)
SELECT lookup_code, 
       exec proc_code_xref(lookup_code) 
FROM (SELECT DISTINCT lookup_code FROM #workinprogress) AS DistinctValues

-- Update the #workinprogress table using join
UPDATE wp
SET wp.xref = xr.xref_code
FROM #workinprogress wp
JOIN XRef xr ON wp.lookup_code = xr.lookup_code

Key Benefits of This Approach

Improved Performance: Eliminating the cursor means that the database engine can optimize the processing more effectively.
Simplified Code: Using a single update statement makes the logic easier to read and maintain.
Scalability: This approach can be easily modified if the number of distinct lookup codes changes.

Conclusion

By transitioning away from cursors and utilizing a cross-reference table, you can enhance the performance and maintainability of your Sybase T-SQL code. In the world of databases, every bit of optimization counts, especially when dealing with large datasets. Always remember that the cleaner and more straightforward your SQL, the better it will perform.

Final Thoughts

If you find yourself frequently dealing with cursors in your code, consider exploring alternative strategies such as this one. With the right approach, you can ensure your database queries are efficient without losing the clarity of your code.

How to Avoid Using Cursors in Sybase (T-SQL)