Forcing only certain nodes to perform coordination within a cluster

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1

My understanding is the node receiving the REST query will act as a coordinating node.
Is this assumption false?
What I'm trying to do is to force say 3 nodes to handle both ingest and coordinating functions. Those 3 nodes are not data nodes. For the sake of this discussion, we can leave out ingest function because I can simply split it into 3 new nodes.

From my experience, the CPU pressure is not on the 3 nodes receiving query when I'm performing CPU intensive aggregations.

If my setup is wrong, how can I achieve forcing coordination from certain none-data nodes?
The purpose is pretty simple. Have data nodes only to gather data. Leave aggregation to none-data nodes.
This way, I can scale them differently based on query need.

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No, that is correct.

That is expected as a lot of the processing is performed by the nodes holding the data, before processed results are send to the coordinating node for final processing and aggregation.

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Thanks for the quick response.
Can you please help answer this question?
I have a query which results in 7K document hits (very small in the index with billions of documents).
I then perform 2 layer aggregation. The search portion returns very quickly and consistently. But the aggr takes about 3 times longer.
If the gathering phase yields only 7K documents (across say 5 nodes/shards), shouldn't the aggregation phase be very quick as well?

we have noticed a similar aggregation on a different smaller index seems to be much faster. This suggests the total number of documents in the index plays a role. If it does, why? Shouldn't the filtering phase reduce the result where aggregation time should be fast?

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