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https://issues.apache.org/jira/browse/ARROW-9937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17191993#comment-17191993
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Jorge commented on ARROW-9937:
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[~andygrove], I remember that you wanted to touch this. If not, let me know and
I take a shoot at it.
Looking at [Ballista's source code for
this|https://github.com/ballista-compute/ballista/blob/main/rust/ballista/src/execution/operators/hash_aggregate.rs]
, I think that we have the same issue there. :/
> [Rust] [DataFusion] Average is not correct
> ------------------------------------------
>
> Key: ARROW-9937
> URL: https://issues.apache.org/jira/browse/ARROW-9937
> Project: Apache Arrow
> Issue Type: Bug
> Components: Rust, Rust - DataFusion
> Reporter: Jorge
> Priority: Major
>
> The current design of aggregates makes the calculation of the average
> incorrect.
> It also makes it impossible to compute the [geometric
> mean|https://en.wikipedia.org/wiki/Geometric_mean], distinct sum, and other
> operations.
> The central issue is that Accumulator returns a `ScalarValue` during partial
> aggregations via {{get_value}}, but very often a `ScalarValue` is not
> sufficient information to perform the full aggregation.
> A simple example is the average of 5 numbers, x1, x2, x3, x4, x5, that are
> distributed in batches of 2, {[x1, x2], [x3, x4], [x5]}. Our current
> calculation performs partial means, {(x1+x2)/2, (x3+x4)/2, x5}, and then
> reduces them using another average, i.e.
> {{((x1+x2)/2 + (x3+x4)/2 + x5)/3}}
> which is not equal to {{(x1 + x2 + x3 + x4 + x5)/5}}.
> I believe that our Accumulators need to pass more information from the
> partial aggregations to the final aggregation.
> We could consider taking an API equivalent to
> [spark](https://docs.databricks.com/spark/latest/spark-sql/udaf-scala.html),
> i.e. have an `update`, a `merge` and an `evaluate`.
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