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The read mode determines how the OLAP processor retrieves data duringnavigation. There are three different data-retrieval methods: This mode transfers the smallest amount of data from the database tothe OLAP processor. However, it is also the mode that has the mostread-cycles. In the "Read During Navigation" mode described below, the systemrequests the data for the fully expanded hierarchy. In the "Read DuringNavigation / Hierarchy Expansion" mode, however, the databaseaggregates the data along the hierarchy and transfers it to the OLAPprocessor at the lowest level of the hierarchy that is displayed in thestart list. When a hierarchy node is expanded, the system is able totarget the lower-level nodes directly. You can improve the performance of queries that have large displayhierarchies, by creating aggregates on an intermediate level of thehierarchy, greater than or equal to the start level. Depending on the navigation status of the query in the BusinessExplorer, the OLAP processor requests only the data that is required.The data is read in each step of the navigation. Unlike the "Read During Navigation / Hierarchy Expansion" mode, displayhierarchies are always read completely on the leaf-level. When nodes are expanded, the OLAP processor is able to read the datafrom the main memory. When the database is accessed, the best aggregate tables are alwaysused and aggregated directly on the database whenever possible. In this mode, there is only one actual read. However, when the query isexecuted in the Business Explorer, all the data that is required forall the possible navigation steps in the query is read from the mainmemory of the OLAP processor. During navigation, any new navigationstatuses are aggregated and calculated from the data in the mainmemory. In general, the "Read During Navigation / Hierarchy Expansion" modeperforms better than the other two modes. This is because it requestsonly the data that the user wants to see. Unlike the "Read During Navigation / Hierarchy Expansion" mode, andwith regards to performance, the "Read During Navigation" settingapplies only to queries with display hierarchies. With regards to performance, and in contrast to the other two modes,the "Read All Data at Once" setting also applies to queries with freecharacteristics. Since the OLAP processor transfers the aggregation foreach query view, the aggregate concept (working with pre-aggregateddata) is least supported in the "Read All Data at Once" mode. Use a mode other than "Read During Navigation / Hierarchy Expansion"only in exceptional cases. The "Read All Data at Once" mode, for example, is useful in thefollowing cases:
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