Fact Table Considerations When Merging Partitions
When you merge partitions, the filters of both partitions are combined using OR to create a filter for the resulting partition. This combined filter specifies the set of facts used in the resulting partition. If the source partition and target partition use the same fact table in a data source or data source view, the combined filter specifies the fact table data that is appropriate to the resulting partition. Because the facts that are required for the resulting partition are present in the fact table, no further action is necessary.
Fact tables are not merged automatically when you merge partitions. You must manually merge fact tables when the partitions being merged have different fact tables. When you merge partitions that use different fact tables, the resulting partition refers only to the original fact table of the target partition. You must manually merge the facts from the fact table of the source partition into the fact table of the target partition. Alternatively, you can change the source for the merged partition to a named query that returns the contents of two separate fact tables. If this manual step is not performed, the fact table does not contain complete information.
If the merged partitions are based on two different named queries in the data source view, you must change the named query on which the combined partition is based to include all the rows returned by the separate named queries.
MOLAP Partitions
When multidimensional OLAP (MOLAP) partitions are merged, the facts stored in the multidimensional structures of the partitions are also merged. This results in an internally complete and consistent partition. However, the facts stored in MOLAP partitions are copies of facts in the fact table. When the partition is subsequently processed, the facts in the multidimensional structure are deleted (only for full and refresh) and data is copied from the fact table as specified by the data source and filter for the partition. If the source partition uses a different fact table from the target partition, the fact table of the source partition must be manually merged with the fact table of the target partition to ensure that a complete set of data is available when the resulting partition is processed. This applies as well if the two partitions were based on different named queries.
Warning
A merged MOLAP partition with an incomplete fact table contains an internally merged copy of fact table data and operates correctly until it is processed.
HOLAP and MOLAP Partitions
When hybrid OLAP (HOLAP) or relational OLAP (ROLAP) partitions that have different fact tables are merged, the fact tables are not automatically merged. Unless the fact tables are manually merged, only the fact table associated with the target partition is available to the resulting partition. Facts associated with the source partition are not available for drilldown in the resulting partition, and when the partition is processed, aggregations do not summarize data from the unavailable table.
Important
A merged HOLAP or ROLAP partition with an incomplete fact table contains accurate aggregations, but incomplete facts. Queries that refer to missing facts return incorrect data. When the partition is processed, aggregations are computed only from available facts.
The absence of unavailable facts might not be noticed unless a user attempts to drill down to a fact in the unavailable table or executes a query that requires a fact from the unavailable table. Because aggregations are combined during the merge process, queries whose results are based only on aggregations return accurate data, whereas other queries may return inaccurate data. Even after the resulting partition is processed, the missing data from the unavailable fact table may not be noticed, especially if it represents only a small portion of the combined data.
Fact tables can be merged before or after merging the partitions. However, the aggregations will not accurately represent the underlying facts until both operations have been completed. It is recommended that you merge HOLAP or ROLAP partitions that access different fact tables when users are not connected to the cube that contains these partitions.
You can also create a named query in the data source view of the cube to return the combined contents of two different fact tables. You can then change the source for the merged partition to refer to this named query.