Important 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, the filters of both partitions are ORed together to create a filter for the resulting partition. This combined filter specifies the set of facts used in the resulting partition. If the source and target partitions use the same fact table, the combined filter specifies the data in the fact table appropriate to the partition. Because the facts necessary for the combined partition are present in the fact table, no further action is necessary.
When you merge partitions that use different fact tables, the resulting partition will refer only to the target partition’s fact table. You must manually merge the facts from the source partition’s fact table that are used by the source partition into the target partition’s fact table. If this manual step is not performed, the partition will not contain complete information.
Caution A merged MOLAP (multidimensional OLAP) partition with an incomplete fact table contains an internally merged copy of fact table data and will operate correctly until it is processed.
When 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 and data is copied from the fact table as specified by the partition’s filter. If the source partition uses a different fact table from the target partition, the source partition’s fact table must be manually merged with the target partition’s fact table to ensure that a complete set of data is available when the resultant partition is processed.
Caution A merged HOLAP (hybrid OLAP) or ROLAP (relational OLAP) partition with an incomplete fact table contains accurate aggregations, but incomplete facts. Queries that refer to missing facts will return incorrect data. When the partition is processed, aggregations will be computed only from available facts.
When HOLAP or 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 merged partition. Facts associated with the source partition are not available for drilldown in the merged partition, and when the partition is processed, aggregations will not summarize data from the unavailable table.
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 will return accurate data, whereas other queries may return inaccurate data. Even after the merged 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 merged 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 merging HOLAP or ROLAP partitions that access different fact tables be accomplished when users are not connected to the cube containing these partitions.