Cursors vs. Set-Based Queries: When Should You Choose Set-Based Operations for Database Queries?

Set-Based Queries: A Superior Approach to Cursors

Database querying presents developers with the option of utilizing cursors or set-based queries to retrieve and manipulate data. While cursors involve iterating over rows one at a time, set-based queries perform operations on entire tables or subsets in a single statement.

Benefits of Set-Based Queries

The fundamental advantage of set-based queries lies in their inherent parallelizability. Database engines are capable of optimizing set-based operations by distributing them across multiple threads. This parallelization enables efficient processing of massive datasets in a fraction of the time required by cursor-based operations.

In contrast, cursors process data sequentially and are inherently single-threaded. As a result, they are significantly slower for large data volumes.

Example

Consider the following task:

Retrieve all customers with orders placed in the last 30 days.

Cursor-Based Solution:

DECLARE my_cursor CURSOR
FOR
SELECT *
FROM Customers
WHERE order_date >= DATEADD(day, -30, CURRENT_DATE);
OPEN my_cursor;
FETCH NEXT FROM my_cursor INTO @customer;
WHILE (@@FETCH_STATUS = 0)
BEGIN
    -- Process customer data
    FETCH NEXT FROM my_cursor INTO @customer;
END;
CLOSE my_cursor;
DEALLOCATE my_cursor;

Set-Based Equivalent:

SELECT *
FROM Customers
WHERE order_date >= DATEADD(day, -30, CURRENT_DATE);

The set-based query leverages the parallelizability of the database engine to process all qualifying customers simultaneously, resulting in significantly improved performance compared to the cursor-based approach.

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