This operator is used to aggregate and group the input stream.
group_by:An optional list of attributes over which the grouping should occur.
aggregations:A list if elements where each element contains up to four string:
- The name of the aggregate function, e.g. MAX
- The input attribute over which the aggregation should be done
- The name of the output attribute for this aggregation
- The optional type of the output
dumpAtValueCount:This parameter is used in the interval approach. Here a result is generated, each time an aggregation cannot be changed anymore. This leads to fewer results with a larger validity. With this parameter the production rate can be raised. A value of 1 means, that for every new element the aggregation operator receives new output elements are generated. This leads to more results with a shorter validity.
outputPA: This parameter allow to dump partial aggregates instead of evaluted values. The partial aggregates can be send to other aggregation operators and do a final aggregation (e.g. in case of distribution). The input schema of an aggregate operator that read partial aggregates must state a datatype that is a partial aggregated (see example below). Remark: Aggregate has one input and requires ordered input. To combine different parital aggregations e.g. a union operator is needed to reorder the input elements.
The set of aggregate functions is extensible. The following list is in the core Odysseus:
MAX: The maximum element
MIN: The minimum element
AVG: The average element
SUM: The sum of all elements
COUNT: The number of elements
- MEDIAN: The median element
STDDEV: The standard deviation
Some nonstandard aggregations: These should only be used, if you a familiar with them:
FIRST: The first element
LAST: The last element
NTH: The nth element
RATE: The number of elements per time unit
- NEST: Nest the attribute values in a list
- COMPLETENESS: Ratio of NULL-value elements to number of elements