Oracle Built−in Packages- P56: Ah, for the good old days of Version of PL /SQL! Life was so simple then. No stored procedures or functions and certainly no packages. You had your set of built−in functions, like SUBSTR and TO_DATE. You had the IF statement and various kinds of loops. With these tools at hand, you built your batch−processing scripts for execution in SQL*Plus, and you coded your triggers in SQL*Forms , and you went home at night content with a good day's work done. | Appendix A What s on the Companion Disk The default navigation through a queue for dequeuing is next message. This means that each subsequent dequeue retrieves messages from the queue based on the snapshot or view of the queue as it appeared when the first dequeue was performed. This approach offers a read consistency model similar to that of the Oracle RDBMS. For example if you enqueue a message after you have issued a dequeue command that message will not be dequeued or even checked for availability for dequeuing until all messages already in the queue have been processed. Specifying for message navigation the default is often sufficient for dequeue operations. If either of the following conditions occur however you may need to change the navigation You enqueue messages after dequeues have occurred and you want those newly enqueued messages to be immediately considered for a dequeue. You have a nondefault . priority-based ordering in your queue. In this case if a higher-priority message enters the queue at any time you want to consider that message for dequeuing immediately. In either of these situations you should set the navigation method as follows DECLARE queueopts BEGIN When you use first message navigation you tell Oracle AQ that you want it to consider the entire set of messages in the queue for every dequeue command. Internally this means that AQ will create a new snapshot view of the queue whenever you request a dequeue. You will find many illustrations of the use of first message navigation in the Section section. Dequeuing with message grouping When you create a queue table specifying for the message_ grouping argument any messages enqueued in the same transaction are considered part of a group of messages. This group may consist of only one message there is also no upper limit on the number of messages allowed in a group.
