arrow_extendr

Pass arrow-rs objects to and from an R session

Motivating Example

Say we have the following DBI connection which we will send requests to using arrow. The result of dbGetQueryArrow() is a nanoarrow_array_stream. We want to count the number of rows in each batch of the steam using Rust.

# adapted from https://github.com/r-dbi/DBI/blob/main/vignettes/DBI-arrow.Rmd

library(DBI)
con <- dbConnect(RSQLite::SQLite())
data <- data.frame(
  a = runif(10000, 0, 10),
  b = rnorm(10000, 4.5),
  c = sample(letters, 10000, TRUE)
)

dbWriteTable(con, "tbl", data)

We can write an extendr function which creates an ArrowArrayStreamReader from an &Robj. In the function we instantiate a counter to keep track of the number of rows per chunk. For each chunk we print the number of rows.

#[extendr]
/// @export
fn process_stream(stream: Robj) -> i32 {
    let rb = ArrowArrayStreamReader::from_arrow_robj(&stream)
        .unwrap();

    let mut n = 0;

    rprintln!("Processing `ArrowArrayStreamReader`...");
    for chunk in rb {
        let chunk_rows = chunk.unwrap().num_rows();
        rprintln!("Found {chunk_rows} rows");
        n += chunk_rows as i32;
    }

    n
}

With this function we can use it on the output of dbGetQueryArrow() or other Arrow related DBI functions.

query <- dbGetQueryArrow(con, "SELECT * FROM tbl WHERE a < 3")
process_stream(query)
#> Processing `ArrowArrayStreamReader`...
#> Found 256 rows
#> Found 256 rows
#> Found 256 rows
#> ... truncated ...
#> Found 256 rows
#> Found 256 rows
#> Found 143 rows
#> [1] 2959