# Selective use of duckplyr

This vignette demonstrates how to use duckplyr selectively, for individual data frames or for other packages.

    library(conflicted)
    library(dplyr)
    conflict_prefer("filter", "dplyr")
    #> [conflicted] Will prefer dplyr::filter over any other package.

## Introduction

The default behavior of duckplyr is to enable itself for all data frames in the session. This happens when the package is attached with `library(duckplyr)`, or by calling `methods_overwrite()`. To enable duckplyr for individual data frames instead of session-wide, it is sufficient to prefix all calls to duckplyr functions with `duckplyr::` and not attach the package. Alternatively, `methods_restore()` can be called to undo the session-wide overwrite after `library(duckplyr)`.

## External data with explicit qualification

The following example uses `duckplyr::as_duckdb_tibble()` to convert a data frame to a duckplyr frame and to enable duckplyr operation.

    lazy <-
      duckplyr::flights_df() |>
      duckplyr::as_duckdb_tibble() |>
      mutate(inflight_delay = arr_delay - dep_delay) |>
      summarize(
        .by = c(year, month),
        mean_inflight_delay = mean(inflight_delay, na.rm = TRUE),
        median_inflight_delay = median(inflight_delay, na.rm = TRUE),
      ) |>
      filter(month <= 6)

The result is a tibble, with its own class.

    class(lazy)
    #> [1] "duckplyr_df" "tbl_df"      "tbl"         "data.frame"

    names(lazy)
    #> [1] "year"                  "month"                 "mean_inflight_delay"
    #> [4] "median_inflight_delay"

DuckDB is responsible for eventually carrying out the operations. Despite the filter coming very late in the pipeline, it is applied to the raw data.

    lazy |>
      explain()
    #> ┌---------------------------┐
    #> │       HASH_GROUP_BY       │
    #> │    --------------------   │
    #> │          Groups:          │
    #> │             #0            │
    #> │             #1            │
    #> │                           │
    #> │        Aggregates:        │
    #> │          mean(#2)         │
    #> │         median(#3)        │
    #> │                           │
    #> │        ~67354 Rows        │
    #> └-------------┬-------------┘
    #> ┌-------------┴-------------┐
    #> │         PROJECTION        │
    #> │    --------------------   │
    #> │            year           │
    #> │           month           │
    #> │       inflight_delay      │
    #> │       inflight_delay      │
    #> │                           │
    #> │        ~67355 Rows        │
    #> └-------------┬-------------┘
    #> ┌-------------┴-------------┐
    #> │         PROJECTION        │
    #> │    --------------------   │
    #> │            year           │
    #> │           month           │
    #> │       inflight_delay      │
    #> │                           │
    #> │        ~67355 Rows        │
    #> └-------------┬-------------┘
    #> ┌-------------┴-------------┐
    #> │           FILTER          │
    #> │    --------------------   │
    #> │ (CAST(month AS DOUBLE) <= │
    #> │            6.0)           │
    #> │                           │
    #> │        ~67355 Rows        │
    #> └-------------┬-------------┘
    #> ┌-------------┴-------------┐
    #> │     R_DATAFRAME_SCAN      │
    #> │    --------------------   │
    #> │      Text: data.frame     │
    #> │                           │
    #> │        Projections:       │
    #> │            year           │
    #> │           month           │
    #> │         dep_delay         │
    #> │         arr_delay         │
    #> │                           │
    #> │        ~336776 Rows       │
    #> └---------------------------┘

All data frame operations are supported. Computation happens upon the first request.

    lazy$mean_inflight_delay
    #> [1] -2.673124 -3.855519 -5.147220 -9.370201 -7.356713 -4.244284

After the computation has been carried out, the results are preserved and available immediately:

    lazy
    #> # A duckplyr data frame: 4 variables
    #>    year month mean_inflight_delay median_inflight_delay
    #>   <int> <int>               <dbl>                 <dbl>
    #> 1  2013     4               -2.67                    -5
    #> 2  2013     1               -3.86                    -5
    #> 3  2013     2               -5.15                    -6
    #> 4  2013     5               -9.37                   -10
    #> 5  2013     3               -7.36                    -9
    #> 6  2013     6               -4.24                    -7

## Restoring dplyr methods

The same can be achieved by calling `methods_restore()` after `library(duckplyr)`.

    library(duckplyr)
    #> ✔ Overwriting dplyr methods with duckplyr methods.
    #> ℹ Turn off with `duckplyr::methods_restore()`.

    methods_restore()
    #> ℹ Restoring dplyr methods.

If the input is a plain data frame, duckplyr is not involved.

    flights_df() |>
      mutate(inflight_delay = arr_delay - dep_delay) |>
      explain()
    #> Error in UseMethod("explain"): no applicable method for 'explain' applied to an object of class "c('tbl_df', 'tbl', 'data.frame')"

## Own data

Construct duckplyr frames directly with `duckdb_tibble()`:

    data <- duckdb_tibble(
      x = 1:3,
      y = 5,
      z = letters[1:3]
    )
    data
    #> # A duckplyr data frame: 3 variables
    #>       x     y z
    #>   <int> <dbl> <chr>
    #> 1     1     5 a
    #> 2     2     5 b
    #> 3     3     5 c

## In other packages

Like other dependencies, duckplyr must be declared in the `DESCRIPTION` file and optionally imported in the `NAMESPACE` file. Because duckplyr does not import dplyr, it is necessary to import both packages. The recipe below shows how to achieve this with the usethis package.

- Add dplyr as a dependency with `usethis::use_package("dplyr")`
- Add duckplyr as a dependency with `usethis::use_package("duckplyr")`
- In your code, use a pattern like `data |> duckplyr::as_duckdb_tibble() |> dplyr::filter(...)`
- To avoid the package prefix and simply write `as_duckdb_tibble()` or `filter()`:
  - Import the duckplyr function with `usethis::use_import_from("duckplyr", "as_duckdb_tibble")`
  - Import the dplyr function with `usethis::use_import_from("dplyr", "filter")`

Learn more about prudence in `vignette("prudence")`, about fallbacks to dplyr in `vignette("fallback")`, about the translation employed by duckplyr in `vignette("limits")`, about direct use of DuckDB functions in `vignette("duckdb")`, and about the usethis package at https://usethis.r-lib.org/.