Adam Black created ARROW-18102:
----------------------------------
Summary: dplyr::count and dplyr::tally implementation return NA
instead of 0
Key: ARROW-18102
URL: https://issues.apache.org/jira/browse/ARROW-18102
Project: Apache Arrow
Issue Type: Bug
Components: R
Environment: Arrow R package 9.0.0 on Mac OS 12.6 with R 4.2.0
Reporter: Adam Black
I'm using dplyr with FileSystemDataset objects. The expected behavior is
similar (or the same as) dataframe behavior. When the FileSystemDataset has
zero rows dplyr::count and dplyr::tally return NA instead of 0. I would expect
the result to be 0.
``` r
library(arrow)
#>
#> Attaching package: 'arrow'
#> The following object is masked from 'package:utils':
#>
#> timestamp
library(dplyr)
#>
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#>
#> filter, lag
#> The following objects are masked from 'package:base':
#>
#> intersect, setdiff, setequal, union
path <- tempfile(fileext = ".feather")
zero_row_dataset <- cars %>% filter(dist < 0)
# expected behavior
zero_row_dataset %>%
count()
#> n
#> 1 0
zero_row_dataset %>%
tally()
#> n
#> 1 0
nrow(zero_row_dataset)
#> [1] 0
# now test behavior with a FileSystemDataset
write_feather(zero_row_dataset, path)
ds <- open_dataset(path, format = "feather")
ds
#> FileSystemDataset with 1 Feather file
#> speed: double
#> dist: double
#>
#> See $metadata for additional Schema metadata
# actual behavior
ds %>%
count() %>%
collect() # incorrect result
#> # A tibble: 1 × 1
#> n
#> <int>
#> 1 NA
ds %>%
tally() %>%
collect() # incorrect result
#> # A tibble: 1 × 1
#> n
#> <int>
#> 1 NA
nrow(ds) # works as expected
#> [1] 0
```
<sup>Created on 2022-10-19 with [reprex
v2.0.2](https://reprex.tidyverse.org)</sup>
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