Oracle Distributed Systems

Refresh Groups

As in the previous example, often you need snapshots on a group of related tables, which may have interdependencies. If you refresh snapshots individually, the resulting data is not guaranteed to have a point-in-time consistency; in fact, it probably will not. For example, if we were to refresh the ORDER_ITEMS snapshot followed by the ORDERS table, we could end up with entries in ORDERS that have no corresponding entries in ORDER_ITEMS if users create orders while the ORDER_ITEMS table refreshes. Obviously, this is not an acceptable state of affairs.

Oracle uses theconcept of a refresh group to encapsulate snapshots that must have point-in-time consistency. You are guaranteed that all snapshots in a single refresh group will be refreshed with data from the master tables as of a single point in time. In addition, refresh groups provide ease of management because Oracle includes a variety of built-in procedures in the package DBMS_REFRESH for their maintenance. The following procedures manipulate refresh groups at the snapshot site:

DBMS_REFRESH.ADD

Adds a snapshot to an existing group.

DBMS_REFRESH. CHANGE

Modifies properties of a refresh group, such as refresh interval, next refresh time, and so on.

DBMS_REFRESH.DESTROY

Drops a refresh group.

DBMS_REFRESH. MAKE

Creates a refresh group.

DBMS_REFRESH. SUBTRACT

Removes a snapshot from an existing refresh group.

The following sections provide brief examples of using these procedures; refer to Appendix A, for the complete API ...