allow named arguments to 'pass through' in translation to SQL?

[cboettig]

Most of the time dbplyr is very good about letting functions known to the database backend (e.g. duckdb with spatial extension), to just "pass through", e.g. if tbl is a lazy table to a duckdb extension, we can do:

tbl |> 
  mutate(geom = st_transform(geom, "EPSG:4326", "EPSG:3857")) 

because st_transform() is understood by duckdb.

(Full reprex here: cboettig/duckdbfs#34).

But in this particular case, DuckDB requires an extra argument for working with EPSG:4326, as described in the docs. We want the SQL query to render as:

SELECT ST_TRANSFORM(geom,  'EPSG:4326',  'EPSG:3857', always_xy := true) AS geom

But I cannot see how to get the always_xy := true part to render properly. E.g. if we try this:

tbl |> 
  mutate(geom = st_transform(geom, "EPSG:4326", "EPSG:3857", always_xy = TRUE)) 

dbplyr gives us this kinda nonsense SQL instead:

SELECT ST_TRANSFORM(geom,  'EPSG:4326',  'EPSG:3857',  TRUE AS always_xy) AS geom

The docs just suggest that "unknown functions translate "as is" but that does not seem to be what happens here.

Thanks!

[ejneer]

Not immediately clear how this could be generally addressed without invasive changes. But, aside from having to "teach" dbplyr about each particular function by defining them in an appropriate backend, I think you can get there with inlining some sql. e.g.:

pad_tract |>
  select(STATE, FIPS, geom) |>
  filter(STATE == "California") |> 
  distinct() |>
  mutate(geom = st_transform(geom, "EPSG:4326", "EPSG:3857", sql("always_xy := TRUE"))) |> show_query()

returning:

SELECT
  STATE,
  FIPS,
  st_transform(geom, 'EPSG:4326', 'EPSG:3857', always_xy := TRUE) AS geom
FROM (
  SELECT DISTINCT STATE, FIPS, geom
  FROM ecwqshpcycckzfg
  WHERE (STATE = 'California')
) q01

[cboettig]

@ejneer Very clever! I think this is a good enough escape hatch for me!