GH-49340: [R] Preserve row order in write_dataset()

[marberts]

Rationale for this change

write_dataset(df) need not preserve the row-ordering of df across partitions. The arrow C++ library was recently updated (since 21.0.0) so that row ordering can be preserved when writing across partitions. This is useful for cases where it is assumed that row-ordering is unchanged within each partition.

df <- tibble::tibble(x = 1:1.5e6, g = rep(1:15, each = 1e5))

df |>
  dplyr::group_by(g) |>
  arrow::write_dataset("test1", preserve_order = FALSE)

df |>
  dplyr::group_by(g) |>
  arrow::write_dataset("test2", preserve_order = TRUE)

test1 <- arrow::open_dataset("test1") |>
  dplyr::collect()

test2 <- arrow::open_dataset("test2") |>
  dplyr::collect()

# Current behavior.
all.equal(test1 |> sort_by(~ g), df)
#> [1] "Component \"x\": Mean relative difference: 0.0475804"

# Preserve order.
all.equal(test2 |> sort_by(~ g), df)
#> [1] TRUE

^{Created on 2026-02-20 with reprex v2.1.1}

What changes are included in this PR?

Added an argument preserve_order to write_dataset() that sets FileSystemDatasetWriteOptions.preserve_order to true in the call to ExecPlan_Write().

Are these changes tested?

Partially. The change is small, so I haven't written unit tests. I can revisit this if necessary.

Are there any user-facing changes?

Yes, there is a new argument in write_dataset(). The default keeps the current behavior and the argument appears after all existing arguments, so the change in backwards compatible.

• GitHub Issue: [R] Preserve row order in write_dataset() #49340

[github-actions]

⚠️ GitHub issue #49340 has been automatically assigned in GitHub to PR creator.

[jonkeane]

Thanks for the contribution!

Would you mind please writing some tests for this behavior? Somewhere in https://github.com/apache/arrow/blob/main/r/tests/testthat/test-dataset-write.R (+ following similar patterns there) would be lovely.

[jonkeane]

Thanks for the tests, I have some suggestions about naming + slightly more idiomatic expectations.

It also looks like there are some cpp linting issues too: https://github.com/apache/arrow/actions/runs/22290480080/job/64535896409?pr=49343#step:6:42