How Does MySQL Process order by and limit in a Query?

When a query with an ORDER BY and LIMIT clause is executed in MySQL, the following steps are taken:

• The query is parsed and optimized by the MySQL query optimizer.
• The optimizer determines the best execution plan for the query, which includes selecting the appropriate indexes and join methods.
• The query is executed, and the result set is sorted according to the ORDER BY clause.
• The LIMIT clause is applied to the sorted result set, and only the specified number of rows are returned.

There are multiple methods to optimize the processing of ORDER BY and LIMIT clauses in MySQL:

• Using Indexes: If the ORDER BY clause is based on a column that has an index, MySQL can use the index to sort the result set. This can significantly improve the performance of the query. For example, consider the following query:

   SELECT * FROM orders ORDER BY order_date LIMIT 10;

If the order_date column has an index, MySQL can use the index to sort the result set and return only the first 10 rows.

• Using Temporary Tables: If the ORDER BY clause is based on a complex expression or involves multiple tables, MySQL may need to create a temporary table to sort the result set. This can be a resource-intensive operation, especially for large result sets. To optimize this process, MySQL can use an in-memory temporary table instead of a disk-based table. For example, consider the following query:

   SELECT * FROM orders o JOIN customers c ON o.customer_id = c.customer_id ORDER BY c.last_name, o.order_date LIMIT 10;

MySQL may need to create a temporary table to sort the result set based on the last_name column from the customers table and the order_date column from the orders table. By default, MySQL uses a disk-based temporary table for this operation. However, you can configure MySQL to use an in-memory temporary table by setting the tmp_table_size and max_heap_table_size system variables.

• Using Query Optimization Techniques: MySQL uses various query optimization techniques to improve the performance of queries with ORDER BY and LIMIT clauses. For example, MySQL can use a loose index scan to avoid sorting the entire result set. This technique involves scanning the index in the order specified by the ORDER BY clause and returning only the first few rows that match the WHERE clause. For example, consider the following query:

   SELECT * FROM orders WHERE order_date >= '2021-01-01' ORDER BY order_date LIMIT 10;

If the order_date column has an index, MySQL can use a loose index scan to return only the first 10 rows that match the WHERE clause and are sorted by the order_date column. This can be much faster than sorting the entire result set.