Why do people hate SQL cursors so much?

sql

best-practices

performance

join

byAlex Kataev·Sep 9, 2024

SQL cursors are often tagged as inefficient because they handle one row at a time, rather than harnessing the full set-based processing power of the database. This can lead to slower execution and greater resource consumption. A set-based operation accomplishes the same task more swiftly and effectively by processing data in bulk:

-- Our old friend Mr. Cursor...
DECLARE @ID INT;
DECLARE myCursor CURSOR FOR SELECT ID FROM MyTable WHERE Condition = 1;
OPEN myCursor;
FETCH NEXT FROM myCursor INTO @ID; -- Fetching, always fetching...
WHILE @@FETCH_STATUS = 0
BEGIN
    -- Some operations on @ID, tedious as an Oscar thank-you speech
    FETCH NEXT FROM myCursor INTO @ID; // More fetching...such exercise!
END;
CLOSE myCursor; -- Finally we can chill!
DEALLOCATE myCursor; -- Sayonara!

-- Introducing, the sleek, speedy, set-based operation!
UPDATE MyTable SET Column = NewValue WHERE Condition = 1;

The set-based operation here updates all matching rows simultaneously using the built-in optimizations of the relational database, executing operations in a single transaction.

Cursor exceptions: When they make sense

Even though set-based solutions in SQL are often favored, there exist specific situations where cursors might be your best bet:

Complex row operations: Situations requiring intricate logic or external function calls for individual rows may necessitate cursor use.
Batching large operations: Cursors help break large-scale operations into manageable segments, avoiding overwhelming system resources.
Dynamic SQL: Cursors can help construct and execute dynamic SQL statements row by row efficiently.

Set-based mindset: The paradigm shift

Transitioning your approach to a set-based mindset demands a significant amount of unlearning, but the effort pays off in superior performance:

Join over loops: Use SQL's JOIN operation to replace nested loops, which are common with cursors.
Window functions: These powerful functions can handle analytical operations usually done with cursors.
Table-valued functions: A more effective way to apply complex logic across a whole dataset.

Modern SQL features & ORM

Object-Relational Mappings (ORMs) and modern SQL features can simplify cursor management, offering improved development efficiency and code clarity:

Cursors handled invisibly: With ORMs, cursor management happens behind the scenes, separating developers from the complexities of cursors.
SQL enhancements: Modern SQL technologies like Common Table Expressions (CTEs) can make complex queries simpler, which were previously only possible using cursors.

Cursors in their essential role

Cursors aren't inherently bad; it's vital to identify when they're an appropriate tool:

Performance trade-off: Cursors can be up to 30 times slower than set-based approaches in large-scale operations.
Historical usage: Cursors were essential in legacy systems where older languages lacked support for list structures.
Contextual suitability: The suitability of cursors can be heavily context-dependent. They can be a lifesaver in certain scenarios.
Resource handling: In a production setting, cursors can manage locked resources better by processing rows without locking an entire table.

Modern alternatives to traditional cursors

Recent advancements have given rise to multiple alternatives to cursors:

Batch operation: Split huge operations into smaller segments to minify resource usage and system impact.
Recursive CTEs: Using recursive queries, navigate and manage complex data hierarchies without resorting to cursors.
Sequence objects: For operations requiring an order, use sequence objects or temporary tables to create an iterative scheme.

Justifiable cursor usage

There are still situations where using a cursor is legitimate:

Procedural execution: Some database interactions like invoking a stored procedure for each row require cursors.
Lack of alternatives: When SQL constructs fail to support necessary operations, a cursor might be the only choice.
Learning and debugging: For understanding row-by-row data processing, using cursors can be a great pedagogical tool.