Q:

Is there any established procedure to calculate the size of a multidimensional database before actually building it? How should I consider levels and hierarchies while calculating the size?

A:

Les Barbusinski’s Answer: If by multidimensional database you mean MOLAP hypercubes (a.k.a. MDDBs), then there is no single accepted algorithm for estimating the size of the database. Since MOLAP cube technology is proprietary (i.e., unique to each vendor implementation), it’s impossible to have a single algorithm. It all depends on how the MDDB handles things such as hierarchies, "sparse" values, repeating values, "tokens", indexing, etc. Some MOLAP vendors provide an algorithm for estimating the size of their cubes, while others do not (check your documentation). And some of the algorithms that are provided don’t work very well (i.e., the estimate, though rigorously calculated, can be off by an order of magnitude). That’s one of the disadvantages of MDDBs.

On the other hand, if by multidimensional database you mean a star schema relational database, then estimating the size of the database is done the same as for any other relational database. In a nutshell, you estimate the size of each table as follows:

  • Estimated Row Count (after 1 year) X Average Row Length X 1.nn (where nn is the percentage of freespace allowed in the table)

Similarly, you estimate the size of each index as follows:

  • Estimated Row Count (after 1 year) X Average Index Length X 1.nn (where nn is the percentage of freespace allowed in the index)

Sum up the values for all tables and indexes, and you’ll have a rough estimate of the database size. Please note that I deliberately left out factoring in such parameters as block sizes, extent allocations, allowances for archive logs and rollback segments, etc. Check with your DBA on how to apply such factors to your particular database. These factors – in total – should not affect your overall size estimate by more than 15 percent. Hope this helps.

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