[jira] [Commented] (ARROW-13627) [C++] ScalarAggregateOptions don't make sense (in hash aggregation)

["Gamel Alomaisi (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-13627?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17410867#comment-17410867 ] 

Gamel Alomaisi commented on ARROW-13627:
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Hey hey what’s up 

> [C++] ScalarAggregateOptions don't make sense (in hash aggregation)
> -------------------------------------------------------------------
>
>                 Key: ARROW-13627
>                 URL: https://issues.apache.org/jira/browse/ARROW-13627
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Neal Richardson
>            Assignee: David Li
>            Priority: Major
>              Labels: kernel, pull-request-available, query-engine
>             Fix For: 6.0.0
>
>          Time Spent: 3h
>  Remaining Estimate: 0h
>
> R's aggregation functions have a {{na.rm}} argument that governs how missing data is handled. Assume {{x <- c(1, 2, NA, 3)}}. {{sum(x, na.rm = TRUE) == 6}} and {{sum(x, na.rm = FALSE)}} is {{NA}} because there is at least one missing value. 
> The ScalarAggregateOptions have two options: skip_nulls and min_count. From what I can tell reading the source, in the context of sum(), skip_nulls affects whether each element of the Array is added to "count", and if count < min_count, you get a null value returned. So to get the expected behavior when calling "sum" on an Array, when na.rm = TRUE, we pass skip_nulls = false, min_count = 0. When na.rm = FALSE, we pass skip_nulls = true, min_count = length(x), the reasoning being that you return a null value unless all values are non-null (and count == length). See https://github.com/apache/arrow/blob/master/r/R/compute.R#L125-L130
> This doesn't really work in the query engine, though. We don't know how many rows are in the data to set an appropriate min_count to get the expected behavior--the dataset being queried may have filtering. And when doing hash aggregation, each group may have a different number of rows. 



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