MySQL Stored Procedure Programming

Tuning ORDER and GROUP BY

GROUP BY, ORDER BY, and certain group functions (MAX, MIN, etc.) may require that data be sorted before being returned to the user. You can detect that a sort is required from the Using filesort tag in the Extra column of the EXPLAIN statement output, as shown in Example 21-19.

Example 21-19. Simple SQL that performs a sort

SELECT *
  FROM customers
 ORDER BY contact_surname, contact_firstname

Explain plan
------------

ID=1     Table=customers   Select type=SIMPLE  Access type=ALL
         Rows=101999
         Key=             (Possible=                              )
         Ref=              Extra=Using filesort

If there is sufficient memory, the sort can be performed without having to write intermediate results to disk. However, without sufficient memory, the overhead of the disk-based sort will often dominate the overall performance of the query.

There are two ways to avoid a disk-based sort:

• Create an index on the columns to be sorted. MySQL can then use the index to retrieve the rows in sorted order.

• Allocate more memory to the sort.

These approaches are described in the following sections.

Creating an Index to Avoid a Sort

If an index exists on the columns to be sorted, MySQL can use the index to avoid a sort. For instance, suppose that the following index exists:

    CREATE INDEX i_customer_name ON customers(contact_surname, contact_firstname)

MYSQL can use that index to avoid the sort operation shown in Example 21-19. Example 21-20 shows the output when the index exists; note the absence of the Using filesort tag and that the