arrow

ARROW-9557: [R] Iterating over parquet columns is slow in R

I don't think this is about `shared_ptr_is_null()` as indicated in the jira issue: https://issues.apache.org/jira/browse/ARROW-9557 I guess profvis (or probably the underlying profiler) struggles with that case.

What happens though is that `$ReadTable()` first calls `$GetSchema()`:

```r
ReadTable = function(col_select = NULL) {
      col_select <- enquo(col_select)
      if (quo_is_null(col_select)) {
        shared_ptr(Table, parquet___arrow___FileReader__ReadTable1(self))
      } else {
        all_vars <- shared_ptr(Schema, parquet___arrow___FileReader__GetSchema(self))$names
        indices <- match(vars_select(all_vars, !!col_select), all_vars) - 1L
        shared_ptr(Table, parquet___arrow___FileReader__ReadTable2(self, indices))
      }
    }
```

and that's expensive for some reason:

``` r
library(arrow)
#>
#> Attaching package: 'arrow'
#> The following object is masked from 'package:utils':
#>
#>     timestamp
library(vctrs)
#>
#> Attaching package: 'vctrs'
#> The following objects are masked from 'package:arrow':
#>
#>     field, list_of
library(purrr)

df <- new_data_frame(
  map(set_names(1:4000), ~rnorm(50000))
)
tf <- tempfile()
write_parquet(df, tf)

reader <- ParquetFileReader$create(tf)

parquet___arrow___FileReader__GetSchema <- arrow:::parquet___arrow___FileReader__GetSchema
parquet___arrow___FileReader__ReadColumn <- arrow:::parquet___arrow___FileReader__ReadColumn

system.time({
  for (i in 1:4000) {
    parquet___arrow___FileReader__GetSchema(reader)
  }
})
#>    user  system elapsed
#>  43.809   1.744  47.962

system.time({
  for (i in 1:4000) {
    parquet___arrow___FileReader__ReadColumn(reader, i)
  }
})
#>    user  system elapsed
#>   3.035   2.448  10.606
```

<sup>Created on 2020-09-07 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0.9001)</sup>

So we probably need a more complete R6 wrapper around `parquet::arrow::FileReader`. https://github.com/apache/arrow/blob/master/cpp/src/parquet/arrow/reader.h#L107

As a start, here is `$GetColumn()`

Closes #8122 from romainfrancois/ARROW-9557/ParquetFileReader

Lead-authored-by: Romain Francois <romain@rstudio.com>
Co-authored-by: Romain François <romain@rstudio.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>