我目前正在使用dplyr::tbl与Azure中的SQL数据库进行交互。我喜欢延迟查询求值的能力，直到它需要，并在许多函数中使用它。
我有时需要做的一件事是修改tabl()将数据读取到R环境中的方式。然而，我刚刚注意到一些意想不到的行为，当使用突变来做到这一点。# Load library
library(dplyr)
library(DBI)
library(odbc)
library(purrr)
使用odbc连接数据库
odbc::dbConnect(odbc::odbc(),
Driver = "ODBC Driver 17 for SQL Server",
...)
> class(db_con)
[1] "Microsoft SQL Server"
attr(,"package")
[1] ".GlobalEnv"
> # Azure Database
> # Microsoft SQL Server Version: 12.00.2195
使用mtcar指定测试数据，将第一行替换为NAs。
> mtcars[1,] <- NA
> mtcars_mod <- as_tibble(mtcars)
> head(mtcars_mod)
# A tibble: 6 x 11
mpg cyl disp hp drat wt qsec vs am gear carb
<dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 NA NA NA NA NA NA NA NA NA NA NA
2 21 6 160 110 3.9 2.88 17.0 0 1 4 4
3 22.8 4 108 93 3.85 2.32 18.6 1 1 4 1
4 21.4 6 258 110 3.08 3.22 19.4 1 0 3 1
5 18.7 8 360 175 3.15 3.44 17.0 0 0 3 2
6 18.1 6 225 105 2.76 3.46 20.2 1 0 3 1
> # See what will be mapped
> purrr::map(mtcars_mod, ~dbDataType(db_con, .))
$mpg
[1] "FLOAT"
$cyl
[1] "FLOAT"
$disp
[1] "FLOAT"
$hp
[1] "FLOAT"
$drat
[1] "FLOAT"
$wt
[1] "FLOAT"
$qsec
[1] "FLOAT"
$vs
[1] "FLOAT"
$am
[1] "FLOAT"
$gear
[1] "FLOAT"
$carb
[1] "FLOAT"
>
> # write to database
> dbWriteTable(db_con, "mtcars_mod", mtcars_mod, overwrite = TRUE)
现在数据已经写入数据库，我想把它放回环境中。目前所有变量都是"float">
> # Returns the correct result using
> dbReadTable(db_con, "mtcars_mod") %>% head()
mpg cyl disp hp drat wt qsec vs am gear carb
1 NA NA NA NA NA NA NA NA NA NA NA
2 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
3 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1
4 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
5 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2
6 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1
>
> # using dplyr::tbl - returns the correct result
> dplyr::tbl(db_con, "mtcars_mod") %>% collect()
# A tibble: 32 x 11
mpg cyl disp hp drat wt qsec vs am gear carb
<dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 NA NA NA NA NA NA NA NA NA NA NA
2 21 6 160 110 3.9 2.88 17.0 0 1 4 4
3 22.8 4 108 93 3.85 2.32 18.6 1 1 4 1
4 21.4 6 258 110 3.08 3.22 19.4 1 0 3 1
5 18.7 8 360 175 3.15 3.44 17.0 0 0 3 2
6 18.1 6 225 105 2.76 3.46 20.2 1 0 3 1
7 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4
8 24.4 4 147. 62 3.69 3.19 20 1 0 4 2
9 22.8 4 141. 95 3.92 3.15 22.9 1 0 4 2
10 19.2 6 168. 123 3.92 3.44 18.3 1 0 4 4
# ... with 22 more rows
到目前为止，这一切都很好，但是如果我想在调用collect()之前更改数据读入环境的方式，该怎么办呢?让我们看看…
> # What if changing the format of a column?
> # This completely changes the expression of the variables
> # mpg is correctly aligned as chr, but for remaining variables 0s or other values
> # are returned for missing values and this changes on each call.
>
> (call_1 <- tbl(db_con, "mtcars_mod") %>%
+ mutate(mpg = as.character(mpg)) %>%
+ collect())
# A tibble: 32 x 11
mpg cyl disp hp drat wt qsec vs am gear carb
<chr> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 NA 2.37e-307 2.73e-310 NaN NaN -5.55e303 NaN 2.37e-307 8.49e-314 0 2.12e-314
2 21 6 e+ 0 1.6 e+ 2 110 3.9 2.88e 0 17.0 0 1 e+ 0 4 4 e+ 0
3 22.8 4 e+ 0 1.08e+ 2 93 3.85 2.32e 0 18.6 1 e+ 0 1 e+ 0 4 1 e+ 0
4 21.4 6 e+ 0 2.58e+ 2 110 3.08 3.22e 0 19.4 1 e+ 0 0 3 1 e+ 0
5 18.7 8 e+ 0 3.6 e+ 2 175 3.15 3.44e 0 17.0 0 0 3 2 e+ 0
6 18.1 6 e+ 0 2.25e+ 2 105 2.76 3.46e 0 20.2 1 e+ 0 0 3 1 e+ 0
7 14.3 8 e+ 0 3.6 e+ 2 245 3.21 3.57e 0 15.8 0 0 3 4 e+ 0
8 24.4 4 e+ 0 1.47e+ 2 62 3.69 3.19e 0 20 1 e+ 0 0 4 2 e+ 0
9 22.8 4 e+ 0 1.41e+ 2 95 3.92 3.15e 0 22.9 1 e+ 0 0 4 2 e+ 0
10 19.2 6 e+ 0 1.68e+ 2 123 3.92 3.44e 0 18.3 1 e+ 0 0 4 4 e+ 0
# ... with 22 more rows
>
> (call_2 <- tbl(db_con, "mtcars_mod") %>%
+ mutate(mpg = as.character(mpg)) %>%
+ collect())
# A tibble: 32 x 11
mpg cyl disp hp drat wt qsec vs am gear carb
<chr> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 NA 2.12e-314 2.37e-307 2.12e-314 0 2.12e-314 NaN 2.12e-314 2.37e-307 2.12e-314 8.49e-314
2 21 6 e+ 0 1.6 e+ 2 1.1 e+ 2 3.9 2.88e+ 0 17.0 0 1 e+ 0 4 e+ 0 4 e+ 0
3 22.8 4 e+ 0 1.08e+ 2 9.3 e+ 1 3.85 2.32e+ 0 18.6 1 e+ 0 1 e+ 0 4 e+ 0 1 e+ 0
4 21.4 6 e+ 0 2.58e+ 2 1.1 e+ 2 3.08 3.22e+ 0 19.4 1 e+ 0 0 3 e+ 0 1 e+ 0
5 18.7 8 e+ 0 3.6 e+ 2 1.75e+ 2 3.15 3.44e+ 0 17.0 0 0 3 e+ 0 2 e+ 0
6 18.1 6 e+ 0 2.25e+ 2 1.05e+ 2 2.76 3.46e+ 0 20.2 1 e+ 0 0 3 e+ 0 1 e+ 0
7 14.3 8 e+ 0 3.6 e+ 2 2.45e+ 2 3.21 3.57e+ 0 15.8 0 0 3 e+ 0 4 e+ 0
8 24.4 4 e+ 0 1.47e+ 2 6.2 e+ 1 3.69 3.19e+ 0 20 1 e+ 0 0 4 e+ 0 2 e+ 0
9 22.8 4 e+ 0 1.41e+ 2 9.5 e+ 1 3.92 3.15e+ 0 22.9 1 e+ 0 0 4 e+ 0 2 e+ 0
10 19.2 6 e+ 0 1.68e+ 2 1.23e+ 2 3.92 3.44e+ 0 18.3 1 e+ 0 0 4 e+ 0 4 e+ 0
# ... with 22 more rows
>
> (call_3 <- tbl(db_con, "mtcars_mod") %>%
+ mutate(mpg = as.character(mpg)) %>%
+ collect())
# A tibble: 32 x 11
mpg cyl disp hp drat wt qsec vs am gear carb
<chr> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 NA 0 NaN NaN NaN NaN NaN NaN NaN NaN NaN
2 21 6 160 110 3.9 2.88 17.0 0 1 4 4
3 22.8 4 108 93 3.85 2.32 18.6 1 1 4 1
4 21.4 6 258 110 3.08 3.22 19.4 1 0 3 1
5 18.7 8 360 175 3.15 3.44 17.0 0 0 3 2
6 18.1 6 225 105 2.76 3.46 20.2 1 0 3 1
7 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4
8 24.4 4 147. 62 3.69 3.19 20 1 0 4 2
9 22.8 4 141. 95 3.92 3.15 22.9 1 0 4 2
10 19.2 6 168. 123 3.92 3.44 18.3 1 0 4 4
# ... with 22 more rows
可以看出，在不同的调用中，R环境中的变量表达式在每次调用时都会发生变化，即使我只修改了一列变量类型(从数字到字符)。
此外，当我使用show_query()生成的SQL查询并直接在MSSQL中运行时，问题不存在。这似乎是R和/或dplyr::tbl的直接问题。
> sessionInfo()
R version 4.0.3 (2020-10-10)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 19042)
Matrix products: default
locale:
[1] LC_COLLATE=English_Australia.1252 LC_CTYPE=English_Australia.1252 LC_MONETARY=English_Australia.1252 LC_NUMERIC=C
[5] LC_TIME=English_Australia.1252
attached base packages:
[1] stats graphics grDevices datasets utils methods base
other attached packages:
[1] tibble_3.0.6 keyring_1.1.0 purrr_0.3.4 odbc_1.3.0 DBI_1.1.1 dplyr_1.0.7
loaded via a namespace (and not attached):
[1] Rcpp_1.0.7 rstudioapi_0.13 magrittr_2.0.1 hms_1.0.0 tidyselect_1.1.0 bit_4.0.4 R6_2.5.0
[8] rlang_0.4.10 fansi_0.4.2 blob_1.2.1 tools_4.0.3 utf8_1.1.4 cli_2.3.0 withr_2.4.1
[15] dbplyr_2.1.1 remotes_2.4.0 ellipsis_0.3.2 bit64_4.0.5 assertthat_0.2.1 lifecycle_1.0.0 neon_0.1.20
[22] crayon_1.4.0 BiocManager_1.30.16 vctrs_0.3.8 glue_1.4.2 compiler_4.0.3 pillar_1.6.2 generics_0.1.0
[29] lubridate_1.7.10 renv_0.12.5 pkgconfig_2.0.3
这是ODBC包而不是dplyr的问题。
从1.3.0更新到1.3.2修复了这个问题。请看这里:https://cran.r-project.org/web/packages/odbc/news/news.html