Anyone know anything about OLAP Internals?

Question

I know a bit about database internals. I've actually implemented a small, simple relational database engine before, using ISAM structures on disk and BTree indexes and all that sort of thing. It was fun, and very educational. I know that I'm much more cognizant about carefully designing database schemas and writing queries now that I know a little bit more about how RDBMSs work under the hood.But I don't know anything about multidimensional OLAP data models, and I've had a hard time finding any useful information on the internet.How is the information stored on disk? What data structures comprise the cube? If a MOLAP model doesn't use tables, with columns and records, then... what? Especially in highly dimensional data, what kinds of data structures make the MOLAP model so efficient? Do MOLAP implementations use something analogous to RDBMS indexes?Why are OLAP servers so much better at processing ad hoc queries? The same sorts of aggregations that might take *hours* to process in an ordinary relational database can be processed in milliseconds in an OLTP cube. What are the underlying mechanics of the model that make that possible?

Answer 1

RDBMS and any technology that are row based will always have the same mechanics, and a very similar storage model. Also you are using set theory as the main method for interacting with the data.

When you switch to a column store you are dealing with a radically different model. Instead of storing rows, you are storing columns. And instead of set theory you are leverage a map/reduce solution.

This means certain things like summation of a column are very slow in rdbms because you need to query every column, and very fast in olap because you can just query a single dimension and also split the work across multiple servers.

Finally OLAP data is usually not being updated, which means a lock will never occur which could slow the process further.