r - RPostgres 未设置正确的字符编码

Question

我正在尝试通过RPostgreSQL包从 Postgres DB 获取一些数据。结果 data.frame 中的字符列具有unknown编码：

Encoding(raw_orders$city)
  [1] "unknown" "unknown" "unknown" "unknown" "unknown" "unknown" "unknown" "unknown" "unknown" "unknown" "unknown" "unknown"

尽管我将所有可以想象的编码标志都设置为 UTF-8：

> dbGetQuery(connection_postgres, "SHOW CLIENT_ENCODING")
  client_encoding
1            UTF8

> dbGetQuery(connection_postgres, "SHOW SERVER_ENCODING")
  server_encoding
1            UTF8

> Sys.getlocale()
[1] "en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8"

如何强制所有字符列使用 UTF-8 而无需通过手动设置Encoding()？