About Scheduled Links and Deferred Transactions

At a materialized view site, deferred transactions include changes to updatable materialized views. The deferred transactions are stored at the materialized view site so that they can be sent to the master site and applied to master tables.

A refresh of an updatable materialized view first pushes the deferred transactions at the materialized view site to its master site. Then, the data at the master site is pulled down and applied to the materialized view.

Optionally, you can choose to push the deferred transactions at a regular interval independent of refresh. A scheduled link is a database link with a user-defined schedule to push deferred transactions. A scheduled link determines how a materialized view site sends its deferred transaction queue to its master site. When you create a scheduled link, Oracle Database creates a job in the local job queue to push the deferred transaction queue to the master site.

If the materialized view site has a constant connection to its master site, then you optionally can use a scheduled link to push the deferred transactions to the master site at regular intervals. If the materialized view site is disconnected from its master site for extended periods of time, then it is typically better not to push deferred transactions on a schedule. In this case, it is best to refresh on demand, which also pushes changes to the master site.

After changes made to updatable materialized views are pushed to the master site, they no longer need to be stored at the materialized view site. To keep the size of the deferred transaction queue in check, you should purge successfully completed deferred transactions. These deferred transactions can be purged at regular intervals according to a purge schedule.