The multiway. array aggregation or simply MultiWay method computes a full data cube by using a multidimensional array as its basic data structure. It is a typical MOLAP approach that uses direct array addressing, where distension. values are accessed via the position or index of their corresponding array locations. Hence, Multiway cannot perform any value-based reordering. an optimization technique. A different approach is developed for the array-based cube construction, as follows —
- Partition the array into chunks. A chunk is a subcube that is small enough to fit into the memory available for cube computation. Chunking is a method for dividing an A-dimension array into small n-dimensional chunks, where each chunk is stored as an object on a disk. The chunks are compressed so as to remove wasted space resulting from empty array cells. For instance, “chunk ID + offset” can be used as a cell addressing mechanism to compress a sparse array structure and when searching for cells within a chunk. Such a compression technique is powerful enough to handle sparse cubes, both on disk and in .memory.
- Compute aggregates by visiting (i.e., accessing the values at) cube cells. The order in which cells are visited can be optimized so as to minimize the number of times that each cell must be revisited, thereby reducing memory access and storage costs. The trick is to exploit this ordering so that partial aggregates can be computed simultaneously, and any unnecessary revisiting of cells is avoided. Because this chunking technique involves “overlapping” some of the aggregation computations, it is referred to as multiway array aggregation. It performs simultaneous aggregation, that is, it computes aggregations simultaneously on multiple dimensions.
