13.3.3 Using Scheduling Modifiers - MySQL

For an application that uses MyISAM tables, you can change the priority of statements that retrieve or modify data. This can be useful in situations where the normal scheduling priorities do not reflect the application's requirements.

Consider an application consisting of a logging process that uses INSERT statements to record information in a log table, and a summary process that periodically issues SELECT queries to generate reports from the log table. Normally, the server will give updates to the table priority over retrievals, so at times of heavy logging activity, report generation might be delayed. If the application places high importance on having the summary process execute as quickly as possible, it can use scheduling modifiers to alter the usual query priorities. Two approaches are possible:

• To elevate the priority of the summary queries, use SELECT HIGH_PRIORITY rather than SELECT with no modifier. This will move the SELECT ahead of pending INSERT statements that have not yet begin to execute.

• To reduce the priority of record logging statements, use INSERT with either the LOW_PRIORITY or DELAYED modifier.

Scheduling modifiers are covered further in section 14.3.2, "Query Scheduling Modifiers."

Please check back next week for the conclusion of this article.