Updated Nov 2011. Check out my latest book on Optimizing SQL Statements for more information. MySQL 5.6.2 also now provides an EXPLAIN syntax for UPDATE and DELETE statements natively.
While most people look at performance optimizations for SELECT statements, UPDATE and DELETE statements are often overlooked. These can benefit from the principles of analyzing the Query Execution Plan (QEP). You can only run an EXPLAIN on a SELECT statement, however it’s possible to rewrite an UPDATE or DELETE statement to perform like a SELECT statement.
To optimize an UPDATE, look at the WHERE clause. If you are using the PRIMARY KEY, no further analysis is necessary. If you are not, it is of benefit to rewrite your UPDATE statement as a SELECT statement and obtain a QEP as previously detailed to ensure optimal indexes are used. For example:
UPDATE t SET c1 = ‘x’, c2 = ‘y’, c3 = 100 WHERE c1 = ‘x’ AND d = CURDATE()
You can rewrite this UPDATE statement as a SELECT statement for using EXPLAIN:
EXPLAIN SELECT c1, c2, c3 FROM t WHERE c1 = ‘x’ AND d = CURDATE()
You should now apply the same principles as you would when optimizing SELECT statements.