Query Read Mode
The read mode for a query can be set in transaction RSRT and in the Hana Modeling Tool:
BW Modeling Tools
The read mode determines how the OLAP processor gets data during navigation. Three values are supported. The mode H has normally a better performance in almost all cases than the other two modes. This is because the system only requests the data that the user wants to see in this mode.
H - Query to be read when you navigate or expand hierarchies
The data across the hierarchy is aggregated and transferred to the OLAP processor on the hierarchy level that is the lowest in the start list. When expanding a hierarchy node, the children of this node are then read. The amount of data transferred from the database to the OLAP processor is the smallest in this mode; however, it has the highest number of read processes.
Recommendation: This is the recommended default setting. In particular, use it if there is a large Hierarchy in the drilldown
X - Query to read data during navigation
The OLAP Engine only requests data that is needed for each navigational state of the query. In contrast to the Mode H, presentation hierarchies are always imported completely on a leaf level here. The OLAP processor can read data from the main memory (local cache) when the nodes are expanded.
Recommendation: Use it for small hierarchies and for large result sets
A - Query to read all data at once
There is only one read process in this mode. When you execute the query, all data that is needed for all possible navigational steps is read from database(or OLAP cache). During navigation, e.g. when you add a free characteristic to the drilldown, the data is retrieved from the main memory. This of cource means that the inicial run of the query may take very long(in case there is no propper cache object yet).
Recommendation: Use it for small InfoCubes and for queries with few free characteristics
Comments to the Usage of the OLAP Cache
If you use the Cache Mode D 'Cache in Database', the cache objects which get created when you run a query depend on the Read Mode. E.g. if mode A was chosen fo a query, a cache object which incudes all navigational steps(apart from changing values of space variables, see SPACE and STATE Variables) is created. This has the advantage that the query can always use the cache object. However, this cache object can become very large in such a case (e.g. when there are many free characteristics)and hence it is only recommneded, if the number of data records transferred from the database (statistics:DBTRANS - see Query Memory Consumption) is not too high in this mode. See also chapter 'Cache Objects / Benefit' in OLAP Cache.