OLAP and its operations

Online Analytical Processing (OLAP) is a technology that lets users interactively analyse multidimensional data (data cubes) from different viewpoints. It is built on the data warehouse and supports fast, complex queries such as aggregation, comparison and trend analysis for decision making. Servers are of types ROLAP, MOLAP and HOLAP.

OLAP operations

        Roll-up (up the hierarchy)
   city -> country        ^
        |                 |
   Drill-down       Slice / Dice / Pivot

Consider a sales cube with dimensions time (quarter), location (city) and item (type).

1. Roll-up (drill-up): aggregates data by climbing a concept hierarchy or by removing a dimension. Example: city → country, so sales of Kathmandu and Pokhara become sales of Nepal.
2. Drill-down (roll-down): the reverse; goes from summary to more detail, by stepping down a hierarchy or adding a dimension. Example: quarter → month.
3. Slice: selects one value on one dimension, giving a sub-cube of one lower dimension. Example: time = "Q1" gives a 2-D location-by-item table.
4. Dice: selects values on two or more dimensions, giving a smaller sub-cube. Example: (location = Kathmandu or Pokhara) and (time = Q1 or Q2) and (item = mobile or laptop).
5. Pivot (rotate): rotates the axes of the view to show the data from a different perspective. Example: swap rows (item) and columns (location).
6. Other: drill-across (query over several fact tables) and drill-through (go to the back-end detail records).