How to spread tbl_dbi and tbl_sql data without downloading to local memory

问题

I am working with large datasets and tidyr's spread usually gives me error messages suggesting failure to obtain memoryto perform the operation.

Therefore, I have been exploring dbplyr. However, as it says here, and also shown below, dbplyr::spread() does not work.

My question here is whether there is another way to accomplish what tidyr::spread does while working with tbl_dbi and tbl_sql data without downloading to local memory.

Using sample data from here, below I present what I get and what I would like to do and get.

#sample tbl_dbi and tbl_sql data

df_sample <- tribble(~group1, ~group2, ~group3, ~identifier, ~value, 
                      8, 24, 6, 'mt_0', 
                      12, 18, 24, 6, 'mt_1', 4)

con <- DBI::dbConnect(RSQLite::SQLite(), "")
df_db <- copy_to(con, df_sample, 'df_sample')

#attempting to spread tbl_dbi and tbl_sql without downloading to local memory

//this does not work

df_db %>% spread(identifier, value)

Error in UseMethod("spread_") : 
  no applicable method for 'spread_' applied to an object of class "c('tbl_dbi', 'tbl_sql', 'tbl_lazy', 'tbl')"

#attempting to spread tbl_dbi and tbl_sql after downloading to local memory

//this spreads the data but the output is in memory
//I would like to keep the output as 'tbl_dbi', 'tbl_sql', and 'tbl_lazy'

df_db %<>% collect() %>% spread(identifier, value)

class(df_db)
[1] "tbl_df"     "tbl"        "data.frame"

Thanks in advance for any help

来源：https://stackoverflow.com/questions/54838395/how-to-spread-tbl-dbi-and-tbl-sql-data-without-downloading-to-local-memory