How Does MySQL Execute ORDER BY and LIMIT Clauses in Queries?

Detailed explanation of the execution order of MySQL’s ORDER BY and LIMIT clauses

Let’s look at a query statement:

<code class="language-sql">SELECT article FROM table1 ORDER BY publish_date LIMIT 20</code>

The goal of this query is to retrieve the 20 most recently published articles from the table1 table. Understanding how MySQL handles ORDER BY and LIMIT clauses is critical to ensuring accurate results.

Execution order

Contrary to popular belief, MySQL does not retrieve all records first and then apply the LIMIT clause. Instead, it follows a specific order of execution:

1. WHERE clause: If the query contains the WHERE clause, MySQL filters records based on the specified conditions.
2. ORDER BY clause: MySQL will sort the records in ascending or descending order according to the ORDER BY clause.
3. LIMIT clause: Finally, MySQL applies the LIMIT clause to retrieve the specified number of records from the sorted results.

Guaranteed accuracy

In our example query, the LIMIT clause will be applied to records that have been sorted by the publish_date field. This means that we are guaranteed to retrieve the 20 most recent articles because the sorting ensures that the most recent articles appear at the top of the sorted results.

Other notes

It is important to note that MySQL also improves performance by using indexes to optimize query execution. If an index exists on the publish_date field, MySQL may use it to sort records efficiently without performing a full table scan.

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