The ability to use materialized view logs during refreshes is only available if
the materialized view??™s base query is simple enough that Oracle can determine which row in the
materialized view will change when a row changes in the source tables.
When a materialized view log is used, only the changes to the master table are sent to the
targets. If you use a complex materialized view, you must use the refresh complete clause in
place of the refresh fast clause. In a complete refresh, the refresh completely replaces the existing
data in the materialized view??™s underlying table.
Materialized view logs must be created in the master database, via the create materialized
view log command. An example of the create materialized view log command is shown here:
create materialized view log on EMPLOYEE
tablespace DATA
storage (initial 500k next 100k pctincrease 0);
The materialized view log is always created in the same schema as the master table.
You can use simple materialized views with materialized view logs to reduce the amount of
network traffic involved in maintaining the replicated data. Because only the changes to the data
will be sent via a materialized view log, the maintenance of simple materialized views should use
fewer network resources than complex materialized views require, particularly if the master tables
are large, fairly static tables. If the master tables are not static, the volume of transactions sent via
the materialized view log may not be any less than would be sent to perform a complete refresh.
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