`as.data.frame(<RPolarsDataFrame>)` seems slow

[eitsupi]

It seems to take 100 times longer than the conversion from arrow::Table.

Could arrow be using ALTREP to make the materialization later?

Details

library(polars)
library(arrow, warn.conflicts = FALSE)

polars_info()
#> Polars R package version : 0.16.3
#> Rust Polars crate version: 0.39.2
#>
#> Thread pool size: 16
#>
#> Features:
#> default                    TRUE
#> full_features              TRUE
#> disable_limit_max_threads  TRUE
#> nightly                    TRUE
#> sql                        TRUE
#> rpolars_debug_print       FALSE
#>
#> Code completion: deactivated
arrow_info()
#> Arrow package version: 15.0.1
#>
#> Capabilities:
#>
#> acero      TRUE
#> dataset    TRUE
#> substrait FALSE
#> parquet    TRUE
#> json       TRUE
#> s3         TRUE
#> gcs        TRUE
#> utf8proc   TRUE
#> re2        TRUE
#> snappy     TRUE
#> gzip       TRUE
#> brotli     TRUE
#> zstd       TRUE
#> lz4        TRUE
#> lz4_frame  TRUE
#> lzo       FALSE
#> bz2        TRUE
#> jemalloc   TRUE
#> mimalloc   TRUE
#>
#> Memory:
#>
#> Allocator jemalloc
#> Current    0 bytes
#> Max        0 bytes
#>
#> Runtime:
#>
#> SIMD Level          avx2
#> Detected SIMD Level avx2
#>
#> Build:
#>
#> C++ Library Version  15.0.1
#> C++ Compiler            GNU
#> C++ Compiler Version 11.4.0

big_df <-  do.call(rbind, lapply(1:5, \(x) nycflights13::flights))

from_r <- bench::mark(
  as_polars_df = as_polars_df(big_df),
  as_arrow_table = as_arrow_table(big_df),
  check = FALSE,
  min_iterations = 5
)

big_pldf <- as_polars_df(big_df)
big_at <- as_arrow_table(big_df)

to_r <- bench::mark(
  pldf = as.data.frame(big_pldf),
  at = as.data.frame(big_at),
  check = FALSE,
  min_iterations = 5
)
#> Warning: Some expressions had a GC in every iteration; so filtering is
#> disabled.

from_r
#> # A tibble: 2 × 6
#>   expression          min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>     <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 as_polars_df      347ms    400ms      2.56    1.07MB    0.639
#> 2 as_arrow_table    357ms    370ms      2.72    1.21MB    0
to_r
#> # A tibble: 2 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 pldf       820.76ms  875.2ms      1.15   192.8MB     1.38
#> 2 at           6.17ms   11.9ms     52.8     13.5MB     5.86

^{Created on 2024-05-06 with reprex v2.1.0}

[eitsupi]

The current implementation of the next branch is much slower.

# Construct an Arrow array from an R vector
long_vec_1 <- 1:10^6

bench::mark(
  arrow = {
    arrow::as_arrow_array(long_vec_1)
  },
  nanoarrow = {
    nanoarrow::as_nanoarrow_array(long_vec_1)
  },
  polars = {
    polars::as_polars_series(long_vec_1)
  },
  neopolars = {
    neopolars::as_polars_series(long_vec_1)
  },
  check = FALSE,
  min_iterations = 5
)
#> # A tibble: 4 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 arrow        2.62ms   2.92ms     328.    19.82MB     2.04
#> 2 nanoarrow  496.13µs 644.87µs    1252.   458.41KB     2.03
#> 3 polars       2.06ms   2.26ms     405.     6.33MB     0
#> 4 neopolars    84.6ms   90.1ms      10.9    1.59MB     0

# Export Arrow data as an R vector
arrow_array_1 <- arrow::as_arrow_array(long_vec_1)
nanoarrow_array_1 <- nanoarrow::as_nanoarrow_array(long_vec_1)
polars_series_1 <- polars::as_polars_series(long_vec_1)
neopolars_series_1 <- neopolars::as_polars_series(long_vec_1)

bench::mark(
  arrow = {
    as.vector(arrow_array_1)
  },
  nanoarrow = {
    as.vector(nanoarrow_array_1)
  },
  polars = {
    as.vector(polars_series_1)
  },
  neopolars = {
    as.vector(neopolars_series_1)
  },
  check = TRUE,
  min_iterations = 5
)
#> # A tibble: 4 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 arrow       13.94µs  15.84µs  46309.      4.59KB     4.63
#> 2 nanoarrow   559.9µs   1.85ms    513.      3.85MB    72.8
#> 3 polars       6.45ms   8.79ms    112.      5.93MB     9.13
#> 4 neopolars  148.82ms 164.65ms      6.02    5.24MB     0

^{Created on 2024-09-05 with reprex v2.1.1}

This is strange because the construction process seems to be almost identical (the main branch branches with or without NA, but in fact the speed does not seem to change with or without NA).

https://github.com/pola-rs/r-polars/blob/f55eade690b6b505b310d05ef66492e5e47ddb68/src/rust/src/conversion_r_to_s.rs#L138-L150 https://github.com/pola-rs/r-polars/blob/72897e5bd1331e1ca9c079b02094e1d326249a71/src/rust/src/series/construction.rs#L22-L28

Is the superior export speed of arrow and nanoarrow probably due to the use of ALTREP?