[GitHub] [arrow] thisisnic commented on a diff in pull request #12799: ARROW-15827: [R] Improve UX of write_dataset(..., max_rows_per_group)

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

thisisnic commented on code in PR #12799:
URL: https://github.com/apache/arrow/pull/12799#discussion_r843723852


##########
r/R/dataset-write.R:
##########
@@ -153,6 +153,16 @@ write_dataset <- function(dataset,
     }
   }
 
+  if (!missing(max_rows_per_file) && max_rows_per_group > max_rows_per_file) {
+    if (!missing(max_rows_per_group)) {
+      warning(paste0(c(
+        "'max_rows_per_group' must be less or equal to 'max_rows_per_file'.",
+        "\n'max_rows_per_group' set to value of 'max_rows_per_file'."

Review Comment:
   ```suggestion
           "`max_rows_per_group` must be less or equal to `max_rows_per_file`.",
           "\n`max_rows_per_group` set to value of `max_rows_per_file`."
   ```
   
   Nice informative warning here.  Typically, we use backticks (```) to refer to parameter values in these kinds of messages.
   
   How about though, we adjust the conditions under which the change in line 163 below is triggered to only include when `max_rows_per_file` has been set (i.e. isn't missing) *and* `max_rows_per_group` hasn't been changed from the default (i.e. is "missing")?  
   
   I'm hesitant to make too many assumptions about user intentions, but I think we could safely say here that it's in those circumstances the user most likely wants to set the maximum rows per file and just hasn't paid attention to the `max_rows_per_group` parameter, and so we can just update the value silently with no warning. 



##########
r/tests/testthat/test-dataset-write.R:
##########
@@ -506,6 +506,47 @@ test_that("Max partitions fails with non-integer values and less than required p
   )
 })
 
+test_that("max_rows_per_group is adjusted if at odds with max_rows_per_file", {
+  skip_if_not_available("parquet")
+  df <- tibble::tibble(
+    int = 1:10,
+    dbl = as.numeric(1:10),
+    lgl = rep(c(TRUE, FALSE, NA, TRUE, FALSE), 2),
+    chr = letters[1:10],
+  )
+  dst_dir <- make_temp_dir()
+
+  # max_rows_per_group unset => pass
+  expect_silent(
+    write_dataset(df, dst_dir, max_rows_per_file = 5)
+  )
+
+  expect_equal(
+    {
+      write_dataset(df, dst_dir, max_rows_per_file = 5)
+      list.files(dst_dir, "part-") %>%
+        length()
+    },
+    2
+  )

Review Comment:
   Great attention to detail, but I think we can remove this test, as it's a little out of scope of this ticket - it's basically testing that `max_rows_per_file` works as intended (which should be tested in the C++ layer anyway), rather than the specific thing this ticket addresses (adjusting the behaviour when the user specified `max_rows_per_file` at odds with `max_rows_per_group`).



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