Handling of unsupported column types in Oracle DB, tbl() vs dbGetQuery()

[michaelgrund]

Maybe related to #369.

When requesting data from an Oracle DB via

library(tidyverse)
library(DBI)
library(ROracle)

drv <- dbDriver("Oracle")

con <- dbConnect(drv, 
                 dbname = "dbname",
                 username = "user",
                 password = "pw"
                 )

from some tables with unsupported column types tbl() does behave different to the approach when using dbGetQuery():

tbl(con, I("SYS.ALL_TAB_COLUMNS")) %>% 
  filter(OWNER == "AR") %>% 
  select(OWNER, TABLE_NAME, COLUMN_NAME)

gives the following error:

Error in `db_query_fields.DBIConnection()`:
! Can't query fields.
ℹ Using SQL: SELECT * FROM SYS.ALL_TAB_COLUMNS "q11" WHERE (0 = 1)
Caused by error in `.oci.GetQuery()`:
! unsupported column type
Backtrace:
  1. tbl(con, I("SYS.ALL_TAB_COLUMNS")) %>% select(OWNER)
  4. dplyr:::tbl.DBIConnection(con, I("SYS.ALL_TAB_COLUMNS"))
  6. dbplyr:::tbl.src_dbi(...)
  7. dbplyr::tbl_sql(c(subclass, "dbi"), src = src, from = from, ...)
 10. dbplyr:::dbplyr_query_fields(src$con, source)
 11. dbplyr:::dbplyr_fallback(con, "db_query_fields", ...)
 13. dbplyr:::db_query_fields.DBIConnection(con, ...)

while

query <- "
SELECT OWNER, TABLE_NAME, COLUMN_NAME
FROM SYS.ALL_TAB_COLUMNS
WHERE OWNER = 'AR'
"

dbGetQuery(con, query)

works as expected.

From the error message I understand that tbl() is doing a select on all columns (*) before slicing only the requested ones. Is this behavior wanted? Of course I don't know the unsupported columns initially and have to do trial and error to find them out but using tbl() seems to be no solution to me for this approach, right?