[ARROW-14778] [C++] mean on a decimal truncates and does not round

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 7.0.0
• Component/s: C++
• Labels: kernel pull-request-available query-engine
• External issue URL: https://github.com/apache/arrow/issues/30313
• Assignee: David Li
• Reporter: Jonathan Keane
• Created: 19/Nov/21 16:31
• Updated: 11/Jan/23 08:42
• Resolved: 24/Nov/21 14:24

Description

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

df <- data.frame(
  x = c(0.1, 0.2, 0.2, 0.2, 0.2)
)

tab <- Table$create(df)

tab %>%
  summarise(mean(x)) %>% 
  collect()
#> # A tibble: 1 × 1
#>   `mean(x)`
#>       <dbl>
#> 1      0.18

tab %>%
  summarise(x = mean(x)) %>% 
  mutate(x = cast(x, decimal(5, 1))) %>% 
  collect()
#> # A tibble: 1 × 1
#>       x
#>   <dbl>
#> 1   0.2

tab %>%
  mutate(x = cast(x, decimal(5, 1))) %>% 
  summarise(x = mean(x)) %>% 
  collect()
#> # A tibble: 1 × 1
#>       x
#>   <dbl>
#> 1   0.1

Issue Links

• links to: GitHub Pull Request #11754