Partial Aggregation Rule for PQPs

[tobodner]

Blocked by

• #739

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This optimization rule searches for AggregateOperatorProxy nodes, and splits the operation into two nodes as follows:

         ...
          |
AggregateOperatorProxy (pipeline-breaker => final aggregation)
          |
AggregateOperatorProxy (no pipeline-breaker => partial aggregatation)
         ...

The lower Aggregate operation can be scaled out across multiple partitions. The final aggregate, however, is a pipeline breaker and must thus be computed by a single worker.