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Collect a query in background

Source code

Description

This doesn’t block the R session as it calls $collect() in a a detached thread. This can also be used via $collect(collect_in_background = TRUE).

Usage

<LazyFrame>$collect_in_background()

Details

This function immediately returns an RThreadHandle. Use <RPolarsRThreadHandle>$is_finished() to see if done. Use <RPolarsRThreadHandle>$join() to wait and get the final result.

It is useful to not block the R session while query executes. If you use <Expr>$map_batches() or <Expr>$map_elements() to run R functions in the query, then you must pass in_background = TRUE in $map_batches() (or $map_elements()). Otherwise, $collect_in_background() will fail because the main R session is not available for polars execution. See also examples below.

Value

RThreadHandle, a future-like thread handle for the task

Examples

library(polars)

# Some expression which does contain a map
expr = pl$col("mpg")$map_batches(
  \(x) {
    Sys.sleep(.1)
    x * 0.43
  },
  in_background = TRUE # set TRUE if collecting in background queries with $map or $apply
)$alias("kml")

# return is immediately a handle to another thread.
handle = pl$LazyFrame(mtcars)$with_columns(expr)$collect_in_background()

# ask if query is done
if (!handle$is_finished()) print("not done yet")

#> [1] "not done yet"

# get result, blocking until polars query is done
df = handle$join()
df

#> shape: (32, 12)
#> ┌──────┬─────┬───────┬───────┬───┬─────┬──────┬──────┬────────┐
#> │ mpg  ┆ cyl ┆ disp  ┆ hp    ┆ … ┆ am  ┆ gear ┆ carb ┆ kml    │
#> │ ---  ┆ --- ┆ ---   ┆ ---   ┆   ┆ --- ┆ ---  ┆ ---  ┆ ---    │
#> │ f64  ┆ f64 ┆ f64   ┆ f64   ┆   ┆ f64 ┆ f64  ┆ f64  ┆ f64    │
#> ╞══════╪═════╪═══════╪═══════╪═══╪═════╪══════╪══════╪════════╡
#> │ 21.0 ┆ 6.0 ┆ 160.0 ┆ 110.0 ┆ … ┆ 1.0 ┆ 4.0  ┆ 4.0  ┆ 9.03   │
#> │ 21.0 ┆ 6.0 ┆ 160.0 ┆ 110.0 ┆ … ┆ 1.0 ┆ 4.0  ┆ 4.0  ┆ 9.03   │
#> │ 22.8 ┆ 4.0 ┆ 108.0 ┆ 93.0  ┆ … ┆ 1.0 ┆ 4.0  ┆ 1.0  ┆ 9.804  │
#> │ 21.4 ┆ 6.0 ┆ 258.0 ┆ 110.0 ┆ … ┆ 0.0 ┆ 3.0  ┆ 1.0  ┆ 9.202  │
#> │ 18.7 ┆ 8.0 ┆ 360.0 ┆ 175.0 ┆ … ┆ 0.0 ┆ 3.0  ┆ 2.0  ┆ 8.041  │
#> │ …    ┆ …   ┆ …     ┆ …     ┆ … ┆ …   ┆ …    ┆ …    ┆ …      │
#> │ 30.4 ┆ 4.0 ┆ 95.1  ┆ 113.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 2.0  ┆ 13.072 │
#> │ 15.8 ┆ 8.0 ┆ 351.0 ┆ 264.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 4.0  ┆ 6.794  │
#> │ 19.7 ┆ 6.0 ┆ 145.0 ┆ 175.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 6.0  ┆ 8.471  │
#> │ 15.0 ┆ 8.0 ┆ 301.0 ┆ 335.0 ┆ … ┆ 1.0 ┆ 5.0  ┆ 8.0  ┆ 6.45   │
#> │ 21.4 ┆ 4.0 ┆ 121.0 ┆ 109.0 ┆ … ┆ 1.0 ┆ 4.0  ┆ 2.0  ┆ 9.202  │
#> └──────┴─────┴───────┴───────┴───┴─────┴──────┴──────┴────────┘