In order to simplify the redundancy definition process, O3 offers a set of predefined redundancy methods. Each redundancy method defines a redundancy policy for the cube. Some methods are completely automatic, and other give more control to the designer. Apart from the predefined methods there is a manual method in which the designer can define the levels one by one.
The redundancy method is the first thing to select to define a redundancy in a cube. Depending on the method selected, in some cases it will be necessary to define some additional parameters.
The redundancy methods available are:
- (none)
- Top Most
- Fixed Level 1
- Fixed Level 2
- Automatic
- Manual
None
In this case no redundancy level is included in the cube. All the queries will be solved on the dimensions base level.
Top Most
The Top Most method includes an only redundancy level in the superior level of all the dimension hierarchies. The format of the included level is the following:
(S1, S2, ..., SN)
where Si is the superior level of no. i dimension, and N is the number of dimensions in the cube.
This method is oriented to efficiently solve the queries on higher level in the cube. It may be useful to accelerate the opening of the initial view in a cube with many records.
Fixed Level 1
The Fixed Level 1 method includes the Top Most level and an additional redundancy level with the following format:
(1, 1, ...., 1)
that is, a redundancy level for the level 1 in each one of the cube dimensions. If any of the dimensions doesn't have level 1 (it has only one level), the redundancy level will be ignored. If the dimensions have 2 levels, the Fixed Level 1 matches the Top Most.
Fixed Level 2
The Fixed Level 2 method includes the Top Most level, the Fixed Level 1 and an additional redundancy level with the following format:
(2,2, ... , 2)
that is a redundancy level for the level 2 in each one of the cube dimensions. If any dimension doesn't have level 2 (it has less than 3 levels), the redundancy level will be ignored. If all the dimensions have 3 levels, the Fixed Level 2 matches the Top Most.
Automatic
The automatic method makes and analysis of the cube content and automatically decides the location of the redundancy levels through algorithms included in O3.
To make this decision the algorithms use an entry parameter that the designer has to give in section "Automatic Redundancy Configuration". The parameter is the maximum number of records that are to be scanned (and added) in the worst of cases to answer a query. This restriction in practice gives a time limit to answer any query. Based on this data and any additional data from the cube (number of levels in the hierarchies, number of elements in the levels and total number of records in the cube) the automatic algorithms will generate the redundancy levels necessary to comply with this rule.
Depending on the moment the automatic redundancy levels are generated, (See Phases) the algorithms can not know the total number of records the cube will have. If this is the case, the algorithm will be based on the information obtained in the stage of metadata generation (building of dimensions and their hierarchies) regarding the number of records in the cube and will base its decisions on this information.
On the other hand, the designer can indicate for this case through the "Estimate total nr. of tuples" option, a total estimate of tuples in the cube, to be used instead of the estimate.
Notes
- The user has to consider that if the value of the maximum number of records to scan in a query is too low, this can cause the generation of too many redundancy levels and increase the size of the cube. Therefore, this redundancy method should be carefully used, controlling the parameter in particular.
Manual
In the manual method the designer has total control over the redundancy levels that will be included in the cube. The designer will have to define the levels one by one, indicating how each one of them is formed (the level for each dimension).
Once the Manual method is selected, the levels should be entered in the "Manual Redundancy Configuration" section through the Add Level button. Once a new level is added, the level number for each dimension should be specified.
The manual method is more flexible that the others, as it permits to adjust the redundancy included in the cube to the reality of the data managed in the best way. On the other hand, it is the one that requires more knowledge from the designer about the redundancy mechanism and the data included in the cube.
Phases
Given a set of redundancy levels generated by any of the previous methods, O3 organizes these levels in phases according to the dependencies among them. The objective of this organization is to make each level use as base level for its calculation the lowest level possible, and prevent all levels to be calculated on the base level.
Given a set of levels, O3 will make a plan composed by a set of phases each one of which will have the levels calculated on the same base level and will make the calculation for these phases.