Selecting the Partitioning Granularity
The key guideline for determining the optimum granularity for the partitions of a partitioned table or join index is the nature of the workloads that most commonly access the object. The higher the number of partitions you define for an object, the more likely an appropriate range query against the table will perform more quickly, given that the partition granularity is such that the Optimizer can eliminate all but one partition or a few partitions.
On the other hand, it is generally best to avoid specifying too fine a partition granularity. For example if query workloads never access data at a granularity of less than one month, there is no benefit to be gained by defining partitions with a granularity of less than one month. Furthermore, unnecessarily fine partition granularity is likely to increase the maintenance load for a partitioned table, which can lead to degradation of overall system performance. In the end, even though too fine a partition granularity itself does not itself introduce performance degradations, the underlying maintenance on such a table can indirectly degrade performance.