Why is My SQL Query Slow When I Add an ORDER BY Clause?

Slow Query with ORDER BY Clause: A Conundrum Unraveled

In this scenario, a SQL query executes swiftly without the ORDER BY clause but encounters a significant slowdown when it is added. The query joins data from multiple tables and returns a list of rows for a specific user.

Explanation of the Solution:

The culprit behind the query's sluggishness lies in the ordering logic. Without the ORDER BY clause, the database can utilize an index for efficient retrieval. However, when an ordering criterion is specified, the index can no longer be employed, resulting in a full table scan.

The proposed solution offers an alternative approach that maintains the indexing capabilities. By enclosing the original query within a derived table, we effectively create a temporary table with the desired result set. This derived table can then be sorted using the ORDER BY clause without affecting the original query's index utilization.

Underlying Mechanism:

The database optimizes queries by creating indexes on frequently accessed columns. These indexes act as efficient maps, enabling direct retrieval of specific data without the need for a full table scan. In this case, the index on the CourseID column can be utilized for rapid retrieval of rows.

When the ORDER BY clause is added, the database must reorganize the retrieved data in the specified order. This operation interferes with the index's efficiency, leading to a full table scan and a noticeable slowdown.

By encapsulating the original query within a derived table, we effectively create a separate result set that is not affected by the subsequent ORDER BY clause. This allows the database to utilize the index for the initial retrieval and then perform the sorting on the derived table, preserving both efficiency and the desired ordering.

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