Editing Online analytical processing (section)

=== Aggregations ===
It has been claimed that for complex queries OLAP cubes can produce an answer in around 0.1% of the time required for the same query on [[OLTP]] relational data.<ref>{{cite web
  | author=MicroStrategy, Incorporated
  | year=1995
  | title=The Case for Relational OLAP
  | url=http://www.cs.bgu.ac.il/~onap052/uploads/Seminar/Relational%20OLAP%20Microstrategy.pdf
  | access-date=2008-03-20
}}</ref><ref>{{cite journal
  |author1=Surajit Chaudhuri  |author2=Umeshwar Dayal
   |name-list-style=amp | title = An overview of data warehousing and OLAP technology
  | journal = SIGMOD Rec.
  | volume = 26
  | issue = 1
  | year = 1997
  | pages = 65
  | doi = 10.1145/248603.248616
  |citeseerx=10.1.1.211.7178
   |s2cid=8125630
 }}</ref> The most important mechanism in OLAP which allows it to achieve such performance is the use of ''aggregations''. Aggregations are built from the fact table by changing the granularity on specific dimensions and aggregating up data along these dimensions, using an [[aggregate function]] (or ''aggregation function''). The number of possible aggregations is determined by every possible combination of dimension granularities.

The combination of all possible aggregations and the base data contains the answers to every query which can be answered from the data.<ref>{{cite journal
  | last1 = Gray | first1 = Jim
  | author1-link = Jim Gray (computer scientist)
  | last2 = Chaudhuri | first2 = Surajit
  | last3 = Layman | first3 = Andrew
  | last4 = Reichart | first4 = Don
  | last5 = Venkatrao | first5 = Murali
  | last6 = Pellow | first6 = Frank
  | last7 = Pirahesh | first7 = Hamid
  | title = Data Cube: {A} Relational Aggregation Operator Generalizing Group-By, Cross-Tab, and Sub-Totals
  | journal = J. Data Mining and Knowledge Discovery
  | volume = 1
  | issue = 1
  | pages = 29–53
  | year = 1997
  | url = http://citeseer.ist.psu.edu/gray97data.html
  | access-date=2008-03-20
| doi = 10.1023/A:1009726021843
  | arxiv = cs/0701155
  | s2cid = 12502175
 }}</ref>

Because usually there are many aggregations that can be calculated, often only a predetermined number are fully calculated; the remainder are solved on demand. The problem of deciding which aggregations (views) to calculate is known as the view selection problem.  View selection can be constrained by the total size of the selected set of aggregations, the time to update them from changes in the base data, or both. The objective of view selection is typically to minimize the average time to answer OLAP queries, although some studies also minimize the update time. View selection is [[NP-complete]]. Many approaches to the problem have been explored, including [[greedy algorithm]]s, randomized search, [[genetic algorithm]]s and [[A* search algorithm]].

Some aggregation functions can be computed for the entire OLAP cube by [[precomputing]] values for each cell, and then computing the aggregation for a roll-up of cells by aggregating these aggregates, applying a [[divide and conquer algorithm]] to the multidimensional problem to compute them efficiently.{{sfn|Zhang|2017|p=1}} For example, the overall sum of a roll-up is just the sum of the sub-sums in each cell. Functions that can be decomposed in this way are called [[decomposable aggregation function]]s, and include <code>COUNT, MAX, MIN,</code> and <code>SUM</code>, which can be computed for each cell and then directly aggregated; these are known as self-decomposable aggregation functions.{{sfn|Jesus|Baquero|Almeida|2011|loc=2.1 Decomposable functions, pp. 3–4}}

In other cases, the aggregate function can be computed by computing auxiliary numbers for cells, aggregating these auxiliary numbers, and finally computing the overall number at the end; examples include <code>AVERAGE</code> (tracking sum and count, dividing at the end) and <code>RANGE</code> (tracking max and min, subtracting at the end). In other cases, the aggregate function cannot be computed without analyzing the entire set at once, though in some cases approximations can be computed; examples include <code>DISTINCT COUNT, MEDIAN,</code> and <code>MODE</code>; for example, the median of a set is not the median of medians of subsets. These latter are difficult to implement efficiently in OLAP, as they require computing the aggregate function on the base data, either computing them online (slow) or precomputing them for possible rollouts (large space).