Verb translation

There are two parts to dbplyr SQL translation: translating dplyr verbs, and translating expressions within those verbs. This vignette describes how entire verbs are translated; vignette("translate-function") descibes how individual expressions within those verbs are translated.

All dplyr verbs generate a SELECT statement. To demonstrate we'll make a temporary database with a couple of tables

library(dplyr) con <- DBI::dbConnect(RSQLite::SQLite(), ":memory:") flights <- copy_to(con, nycflights13::flights) airports <- copy_to(con, nycflights13::airports)

Single table verbs

  • select() and mutate() modify the SELECT clause:

    flights %>% select(contains("delay")) %>% show_query() flights %>% select(distance, air_time) %>% mutate(speed = distance / (air_time / 60)) %>% show_query()

    (As you can see here, the generated SQL isn't always as minimal as you might generate by hand.)

  • filter() generates a WHERE clause:

    flights %>% filter(month == 1, day == 1) %>% show_query()
  • arrange() generates an ORDER BY clause:

    flights %>% arrange(carrier, desc(arr_delay)) %>% show_query()
  • summarise() and group_by() work together to generate a GROUP BY clause:

    flights %>% group_by(month, day) %>% summarise(delay = mean(dep_delay)) %>% show_query()

Dual table verbs

inner_join() SELECT * FROM x JOIN y ON x.a = y.a
left_join() SELECT * FROM x LEFT JOIN y ON x.a = y.a
right_join() SELECT * FROM x RIGHT JOIN y ON x.a = y.a
full_join() SELECT * FROM x FULL JOIN y ON x.a = y.a
semi_join() SELECT * FROM x WHERE EXISTS (SELECT 1 FROM y WHERE x.a = y.a)
anti_join() SELECT * FROM x WHERE NOT EXISTS (SELECT 1 FROM y WHERE x.a = y.a)
setdiff(x, y) SELECT * FROM x EXCEPT SELECT * FROM y

x and y don't have to be tables in the same database. If you specify copy = TRUE, dplyr will copy the y table into the same location as the x variable. This is useful if you've downloaded a summarised dataset and determined a subset of interest that you now want the full data for. You can use semi_join(x, y, copy = TRUE) to upload the indices of interest to a temporary table in the same database as x, and then perform a efficient semi join in the database.

If you're working with large data, it maybe also be helpful to set auto_index = TRUE. That will automatically add an index on the join variables to the temporary table.

Behind the scenes

The verb level SQL translation is implemented on top of tbl_lazy, which basically tracks the operations you perform in a pipeline (see lazy-ops.R). Turning that into a SQL query takes place in three steps:

  • sql_build() recurses over the lazy op data structure building up query objects (select_query(), join_query(), set_op_query() etc) that represent the different subtypes of SELECT queries that we might generate.

  • sql_optimise() takes a pass over these SQL objects, looking for potential optimisations. Currently this only involves removing subqueries where possible.

  • sql_render() calls an SQL generation function (sql_select(), sql_join(), sql_subquery(), sql_semijoin() etc) to produce the actual SQL. Each of these functions is a generic, taking the connection as an argument, so that the details can be customised for different databases.