[GitHub] [arrow] eitsupi commented on a diff in pull request #14175: ARROW-17737: [R] Continue to retain grouping metadata even if ungroup arrow dplyr query

[GitBox <gi...@apache.org>]

eitsupi commented on code in PR #14175:
URL: https://github.com/apache/arrow/pull/14175#discussion_r990536980


##########
r/R/dplyr.R:
##########
@@ -182,7 +182,7 @@ dim.arrow_dplyr_query <- function(x) {
     # Query on in-memory Table, so evaluate the filter
     # Don't need any columns
     x <- select.arrow_dplyr_query(x, NULL)
-    rows <- nrow(compute.arrow_dplyr_query(x))
+    rows <- nrow(as_arrow_table(x))

Review Comment:
   This is because manipulating metadata for a table with no rows will cause the size to be updated to 0 x 0.
   
   
   ``` r
   mtcars |> arrow::arrow_table() |> dplyr::select(NULL) |> arrow::as_arrow_table()
   #> Table
   #> 32 rows x 0 columns
   #>
   #>
   #> See $metadata for additional Schema metadata
   mtcars |> arrow::arrow_table() |> dplyr::select(NULL) |> arrow::as_arrow_table() |> dplyr::ungroup()
   #> Table
   #> 0 rows x 0 columns
   #>
   #>
   #> See $metadata for additional Schema metadata
   ```
   
   <sup>Created on 2022-10-07 with [reprex v2.0.2](https://reprex.tidyverse.org)</sup>
   
   I don't know if this (handling of tables with no rows) is the problem.



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