Multidimensional databases and online analytical processing (OLAP) how it is related?

Question

http://en.wikipedia.org/wiki/Online_Analytical_Processing

How is this two related? How to know that we are dealing with this type of programme?

Answer 1

The two are often conflated but are not exactly equivalent.

A multi-dimensional database - ie. a star schema: http://en.wikipedia.org/wiki/Star_schema

(or arguably also a snowflake schema) is a way of organising data into fact tables and dimension tables - the former typically hold the numeric data (ie. measurements) while the latter hold descriptive data. A star schema may be implemented using relational database technology or using specialised storage formats that have been optimised for manipulating dimensional data.

OLAP is normally implemented using specialised storage formats that have been optimised for manipulating dimensional data, and features precalculation of summarised values.

Both are normally used as part of datawarehousing. OLAP is likely to be implemented where performance from a non-aggregated SQL database is judged to be inadequate for aggregated reporting requirements.

Answer 2

What multidimensional usually means in the context of OLAP systems is actually a database design based on "Dimensional Modelling" or sofwtare that supports dimensionally modelled data.

The word "Multidimensional" used in that sense is not really very informative because any relational database is inherently multidimensional. (A relation being fundamentally an N-dimensional data structure with the number of dimensions limited only by the constraints of software and hardware). Personally therefore I would prefer to avoid the term multidimensional altogether. It is just too ambiguous to be useful.