f0beab4041
This begins to clean up how `PipelineAggregator`s and executed. Previously, we would create the `PipelineAggregator`s on the data nodes and embed them in the aggregation tree. When it came time to execute the pipeline aggregation we'd use the `PipelineAggregator`s that were on the first shard's results. This is inefficient because: 1. The data node needs to make the `PipelineAggregator` only to serialize it and then throw it away. 2. The coordinating node needs to deserialize all of the `PipelineAggregator`s even though it only needs one of them. 3. You end up with many `PipelineAggregator` instances when you only really *need* one per pipeline. 4. `PipelineAggregator` needs to implement serialization. This begins to undo these by building the `PipelineAggregator`s directly on the coordinating node and using those instead of the `PipelineAggregator`s in the aggregtion tree. In a follow up change we'll stop serializing the `PipelineAggregator`s to node versions that support this behavior. And, one day, we'll be able to remove `PipelineAggregator` from the aggregation result tree entirely. Importantly, this doesn't change how pipeline aggregations are declared or parsed or requested. They are still part of the `AggregationBuilder` tree because *that* makes sense. |
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