across support seems very limited

[mgirlich]

the translation of across() seems to be currently quite limited and buggy. Basically, it only seems to work with a single function.

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
library(dbplyr)
#> 
#> Attaching package: 'dbplyr'
#> The following objects are masked from 'package:dplyr':
#> 
#>     ident, sql

lf <- lazy_frame(g = 1, a = 1, b = 2, c = 3)

# works fine
mutate(lf, across(everything(), mean))
#> <SQL>
#> SELECT AVG(`g`) OVER () AS `g`, AVG(`a`) OVER () AS `a`, AVG(`b`) OVER () AS `b`, AVG(`c`) OVER () AS `c`
#> FROM `df`

# works fine
mutate(lf, across(everything(), list(mean = mean)))
#> <SQL>
#> SELECT AVG(`g`) OVER () AS `g`, AVG(`a`) OVER () AS `a`, AVG(`b`) OVER () AS `b`, AVG(`c`) OVER () AS `c`
#> FROM `df`

# self defined functions do not error but also don't get translated
# this is a bit dangerous as one might not notice this directly
mutate(lf, across(everything(), function(x) mean(x)))
#> <SQL>
#> SELECT `g`, `a`, `b`, `c`
#> FROM `df`

# formulas error
mutate(lf, across(everything(), ~ mean(.x)))
#> Error: Unsupported `.fns` for dbplyr::across()

# multiple functions result in an error
mutate(lf, across(everything(), list(mean = mean, sd = sd)))
#> Error: Result 2 must be a single string, not NULL of length 0

^{Created on 2020-11-09 by the reprex package (v0.3.0)}

Session info

devtools::session_info()
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#>  date     2020-11-09                  
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[mgirlich]

Makes sense to me but then an explicit error message would be great. But I think there should be a way to translate "one-liners" like in filter_at():

tbl_lazy(mtcars) %>% 
  dplyr::filter_at(
    dplyr::vars(mpg, cyl),
    dplyr::all_vars(!is.na(.))
  )

I do not see a way to replace the filter_at() with filter() + across() at the moment. It would be great if this were possible.

[hadley]

That will (probably) become something like:

mtcars %>%
  filter(if_all(c(mpg, cy), ~ !is.na(.x))

[alex-m-ffm]

I think @mgirlich is not completely correct with the problem of multiple standard functions in a list. mean, max and min work for me in a list. It appears more to be an issue of specific functions, among them sd.

library(dplyr)
library(dbplyr)

con <- DBI::dbConnect(RSQLite::SQLite(), dbname = ":memory:")

copy_to(con, nycflights13::flights, "flights",
        temporary = FALSE, 
        indexes = list(
          c("year", "month", "day"), 
          "carrier", 
          "tailnum",
          "dest"
        )
)

flights_db <- tbl(con, "flights")

#this works fine
flights_db %>% group_by(carrier, dest) %>% 
  summarise(across(c(dep_delay, arr_delay, air_time, distance),
                   list(mean, max, min), na.rm = T)
            )

# this does not throw an error but also doesn`t evaluate`
flights_db %>% group_by(carrier, dest) %>% 
  summarise(across(c(dep_delay, arr_delay, air_time, distance),
                   sd)
  )

# outside a list sd works fine for each of the variables...
flights_db %>% group_by(carrier, dest) %>% 
  summarise(sd = sd(distance, na.rm = T))

# with sd included in the list I get an error
flights_db %>% group_by(carrier, dest) %>% 
  summarise(across(c(dep_delay, arr_delay, air_time, distance),
                   list(mean, max, min, sd), na.rm = T)
            )
#> Error: Result 4 must be a single string, not NULL of length 0

Would be great if that could be fixed (besides the formula support)!

[hadley]

Minimal reprex:

library(dbplyr)
library(dplyr, warn.conflicts = FALSE)

db <- memdb_frame(x = 1:10, y = 2:11)
# Need to add support for formulas:
db %>% summarise(across(everything(), ~ mean(.x)))
#> Error: Unsupported `.fns` for dbplyr::across()

# Weird failure mode for sd
db %>% summarise(across(everything(), sd))
#> Error: Query contains no columns
db %>% summarise(across(everything(), list(sd)))
#> Error: Result 1 must be a single string, not NULL of length 0

^{Created on 2021-01-21 by the reprex package (v0.3.0.9001)}

[hadley]

I'll leave #525 for the lack of formula support so this issue can focus on the problem with sd, which arises because I forgot to check the return value of find_fun() which can sometimes be NULL.