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https://issues.apache.org/jira/browse/ARROW-14288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17540992#comment-17540992
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Nicola Crane commented on ARROW-14288:
--------------------------------------
Here's an example of {{nrow()}} returning NA when it shouldn't:
{code:java}
library(arrow)
library(dplyr)
mtcars %>%
distinct() %>%
nrow()
#> [1] 32mtcars %>%
arrow_table() %>%
distinct() %>%
nrow()
#> [1] NA
mtcars %>%
arrow_table() %>%
distinct() %>%
collect()
#> # A tibble: 32 × 11
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 21 6 160 110 3.9 2.62 16.5 0 1 4 4
#> 2 21 6 160 110 3.9 2.88 17.0 0 1 4 4
#> 3 22.8 4 108 93 3.85 2.32 18.6 1 1 4 1
#> 4 21.4 6 258 110 3.08 3.22 19.4 1 0 3 1
#> 5 18.7 8 360 175 3.15 3.44 17.0 0 0 3 2
#> 6 18.1 6 225 105 2.76 3.46 20.2 1 0 3 1
#> 7 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4
#> 8 24.4 4 147. 62 3.69 3.19 20 1 0 4 2
#> 9 22.8 4 141. 95 3.92 3.15 22.9 1 0 4 2
#> 10 19.2 6 168. 123 3.92 3.44 18.3 1 0 4 4
#> # … with 22 more rows{code}
> [R] Implement nrow on some collapsed queries
> --------------------------------------------
>
> Key: ARROW-14288
> URL: https://issues.apache.org/jira/browse/ARROW-14288
> Project: Apache Arrow
> Issue Type: Improvement
> Components: R
> Reporter: Neal Richardson
> Priority: Major
> Fix For: 9.0.0
>
>
> collapse() doesn't always mean we can't determine the number of rows. We can
> try to solve some cases:
> * head/tail: compute number of rows, take the smaller of that and the
> head/tail number
> * if filter == TRUE, take the number of rows of .data (which may contain a
> query)
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