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Casting integer -> numeric is not currently possible with SQLlite #171

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DavisVaughan opened this Issue Oct 6, 2018 · 1 comment

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DavisVaughan commented Oct 6, 2018

I'm not exactly sure why this is happening, but using the sql translation of CAST(x as NUMERIC) does not actually cast an integer to a double value in SQLlite.

I would've thought that it would work, because the documentation seems to suggest this would be the expected behavior, but maybe I am reading it wrong.
https://www.sqlite.org/datatype3.html#affname

At the very least, I think an "easy" solution would just be providing a translation for as.double and as.numeric that casts to REAL or DOUBLE rather than the base scalar default of NUMERIC

library(dplyr)
library(DBI)

con <- dbConnect(RSQLite::SQLite(), ":memory:")
ex <- data.frame(x = 1L)
copy_to(con, ex)
ex_tbl <- tbl(con, "ex")

# it is an integer, as expected
ex_tbl
#> # Source:   table<ex> [?? x 1]
#> # Database: sqlite 3.22.0 [:memory:]
#>       x
#>   <int>
#> 1     1

# no conversion done
mutate(ex_tbl, xx = as.double(x))
#> # Source:   lazy query [?? x 2]
#> # Database: sqlite 3.22.0 [:memory:]
#>       x    xx
#>   <int> <int>
#> 1     1     1

# sql maybe incorrect?
mutate(ex_tbl, xx = as.double(x)) %>%
  show_query()
#> <SQL>
#> SELECT `x`, CAST(`x` AS NUMERIC) AS `xx`
#> FROM `ex`

# confirming that it does not work manually
res_int <- dbGetQuery(con, "SELECT CAST(x as NUMERIC) as xx FROM ex")

class(res_int$xx)
#> [1] "integer"

# casting to REAL or DOUBLE works
res_dbl <- dbGetQuery(con, "SELECT CAST(x as REAL) as xx FROM ex")
class(res_dbl$xx)
#> [1] "numeric"

res_dbl2 <- dbGetQuery(con, "SELECT CAST(x as DOUBLE) as xx FROM ex")
class(res_dbl2$xx)
#> [1] "numeric"

Created on 2018-10-06 by the reprex
package
(v0.2.0).

Session info
devtools::session_info()
#> Session info -------------------------------------------------------------
#>  setting  value                       
#>  version  R version 3.5.1 (2018-07-02)
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#>  ui       X11                         
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#>  collate  en_US.UTF-8                 
#>  tz       America/New_York            
#>  date     2018-10-06
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@hadley

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hadley commented Jan 2, 2019

Minimal reprex:

library(dplyr, warn.conflicts = FALSE)

mf <- dbplyr::memdb_frame(x = 1L)

mf %>% transmute(
  x = TYPEOF(x),
  x_num = TYPEOF(as.numeric(x))
)
#> # Source:   lazy query [?? x 2]
#> # Database: sqlite 3.22.0 [:memory:]
#>   x       x_num  
#>   <chr>   <chr>  
#> 1 integer integer

Created on 2019-01-02 by the reprex package (v0.2.1)

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