Set-Based Queries vs. Cursors: Why Are Set-Based Queries More Efficient?

The Merits of Set-Based Queries over Cursors in Database Queries

In database query writing, developers often face a choice between iterating through rows using a cursor or utilizing a single SQL statement. The latter approach, known as set-based queries, offers several advantages over cursors.

Optimization Benefits:

Set-based queries enable the database engine to optimize the operation by leveraging multi-threading. This is akin to the quicksort algorithm, where the list being sorted is divided into chunks and sorted concurrently in separate threads. SQL engines can perform similar optimizations for massive datasets within a single set-based query.

Sequential Nature of Cursors:

In contrast to set-based queries, cursor-based operations must be executed sequentially, limiting their ability to leverage multi-threading. This can significantly impact performance, especially for large datasets.

Example:

To illustrate the difference, consider the following cursor-based solution:

DECLARE cursor CURSOR FOR
  SELECT *
  FROM table_name;
DECLARE @row_count INT = 0;

OPEN CURSOR;
FETCH NEXT FROM CURSOR INTO @row;

WHILE @@FETCH_STATUS = 0
BEGIN
  -- Process row
  SET @row_count = @row_count + 1;
  FETCH NEXT FROM CURSOR INTO @row;
END;

CLOSE CURSOR;
DEALLOCATE CURSOR;

The relational equivalent of this cursor-based solution would be a single SQL statement:

SELECT *
FROM table_name;

The set-based query can be optimized by the database engine, allowing for parallel execution and significantly improved performance, especially for large datasets.

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