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    Type:Sub-task 
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    Resolution: Fixed
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    Priority:L3 - Default 
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    Affects Version/s: None
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    Component/s: backend
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        Not defined
Context:
Back-end ticket for OPT-3085.
AT:
- I can evaluate reports where I can aggregate variable values for a single variable across all process instances of the current filter
- the evaluation is also possible across several definition and/or tenant versions
- the aggregation can be of the following types: SUM, AVG, MIN, MAX, MEDIAN
- the aggregation works only for numeric variable types. All other variable types throw an error during the evaluation
- it's possible to combine the new variable aggregation reports
- the report rest-API is adjusted according to the new changes