Jorge created ARROW-9937:
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Summary: [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
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 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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