Scheduling and Locking Issues
The previous sections have focused primarily on making individual queries faster. MySQL also allows you to affect the scheduling priorities of statements, which may allow queries arriving from several clients to cooperate better so that individual clients aren't locked out for a long time. Changing the priorities can also ensure that particular kinds of queries are processed more quickly. We'll look first at MySQL's default scheduling policy and then see what options are available to you for influencing this policy. For the purposes of this discussion, a client performing a retrieval (a SELECT) is a reader. A client performing an operation that modifies a table (DELETE, INSERT, REPLACE, or UPDATE) is a writer.