How the tbl() is discovering the remote datasource?

tidyverse
1

What is the initial (discovery) query when defining a datasource with the tbl(con, source_table)?
My assumption is that the tbl() is running a SELECT * FROM <source_table> LIMIT <n> query in the background. Can you confirm?

I am asking because I run frequently in the std::bad_alloc error, but the same query composed with glue_sql() can be fetched without any error using the dbGetQuery(). Some of unused columns triggers this error, but the custom made query is working ok because I am specifying the columns manually in the SQL query.

How many times is a query done for this piece of code (without the final collect())?

flights <- tbl(con, "flights") |> 
  select(year:day, dep_delay, arr_delay) |> 
  filter(dep_delay > 240)

(my guess is 2)

Do you have any idea how the columns can be specified with tbl() in order to avoid this std::bad_alloc type error (and still be able to use the flexibility of the dplyr based queries)?

Thank you.

2

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