Issue when using `last()` in a grouped mutate()

[moodymudskipper]

From: https://stackoverflow.com/questions/74677042/how-to-use-last-when-mutating-by-group-with-dbplyr

Are these expected ? I would expect a grouped last() to always return a single value along the window, and I would expect the 2nd case to reliably give me the result I'd have with a local data frame.

library(dbplyr)
library(dplyr, w = F)
remote_data <- memdb_frame(
  grp = c(2, 2, 2, 1, 3, 1, 1),
  win = c("B", "C", "A", "B", "C", "A", "C"),
  id = c(1,3,5,7,2,4,6),
)

remote_data %>% 
  arrange(grp, win) %>% 
  group_by(grp) %>% 
  mutate(last_id = last(id)) %>% 
  ungroup() %>% 
  print() %>% 
  show_query()
#> # Source:     SQL [7 x 4]
#> # Database:   sqlite 3.39.4 [:memory:]
#> # Ordered by: grp, win
#>     grp win      id last_id
#>   <dbl> <chr> <dbl>   <dbl>
#> 1     1 A         4       4
#> 2     1 B         7       7
#> 3     1 C         6       6
#> 4     2 A         5       5
#> 5     2 B         1       1
#> 6     2 C         3       3
#> 7     3 C         2       2
#> <SQL>
#> SELECT
#>   *,
#>   LAST_VALUE(`id`) OVER (PARTITION BY `grp` ORDER BY `grp`, `win`) AS `last_id`
#> FROM `dbplyr_001`
#> ORDER BY `grp`, `win`

remote_data %>% 
  arrange(grp, win) %>% 
  group_by(grp) %>% 
  window_order(win) %>% 
  mutate(last_id = last(id)) %>% 
  ungroup() %>% 
  print() %>% 
  show_query()
#> # Source:     SQL [7 x 4]
#> # Database:   sqlite 3.39.4 [:memory:]
#> # Ordered by: win
#>     grp win      id last_id
#>   <dbl> <chr> <dbl>   <dbl>
#> 1     1 A         4       4
#> 2     1 B         7       7
#> 3     1 C         6       6
#> 4     2 A         5       5
#> 5     2 B         1       1
#> 6     2 C         3       3
#> 7     3 C         2       2
#> <SQL>
#> SELECT *, LAST_VALUE(`id`) OVER (PARTITION BY `grp` ORDER BY `win`) AS `last_id`
#> FROM `dbplyr_001`
#> ORDER BY `grp`, `win`

remote_data %>% 
  arrange(grp, win) %>% 
  group_by(grp) %>% 
  window_order(grp) %>% 
  mutate(last_id = last(id)) %>% 
  ungroup() %>% 
  print() %>% 
  show_query()
#> # Source:     SQL [7 x 4]
#> # Database:   sqlite 3.39.4 [:memory:]
#> # Ordered by: grp
#>     grp win      id last_id
#>   <dbl> <chr> <dbl>   <dbl>
#> 1     1 A         4       6
#> 2     1 B         7       6
#> 3     1 C         6       6
#> 4     2 A         5       5
#> 5     2 B         1       5
#> 6     2 C         3       5
#> 7     3 C         2       2
#> <SQL>
#> SELECT *, LAST_VALUE(`id`) OVER (PARTITION BY `grp` ORDER BY `grp`) AS `last_id`
#> FROM `dbplyr_001`
#> ORDER BY `grp`, `win`

[mgirlich]

I just learned/realised that the default window frame depends on whether ORDER BY is omitted or not:

• no ORDER BY: ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
• with ORDER BY: RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW

Therefore, the current translation works for first() but not for last().

Thanks for opening this issue 😄