Update to filter() behaviour in bcdata v0.4.0

Update to filter() behaviour in bcdata v0.4.0

2022-10-28

This vignette describes a change in {bcdata} v0.4.0 related to using locally-executed functions in a filter() query with bcdc_query_geodata():

When using bcdc_query_geodata() with filter(), many functions are translated to a query plan that is passed to and executed on the server - this includes the CQL Geometry predicates such as INTERESECTS(), CROSSES(), BBOX() etc, as well as many base R functions. However you sometimes want to include a function call in your filter() statement which should be evaluated locally - i.e., it’s an R function (often an {sf} function) with no equivalent function on the server. Prior to version 0.4.0, {bcdata} did a reasonable (though not perfect) job of detecting R functions inside a filter() statement that needed to be evaluated locally. In order to align with recommended best practices for {dbplyr} backends, as of v0.4.0, function calls that are to be evaluated locally now need to be wrapped in local().

For example, say we want to create a bounding box around two points and use that box to perform a spatial filter on the remote dataset, to give us just the set of local greenspaces that exist within that bounding box.

library(sf)
library(bcdata)

two_points <- st_sfc(st_point(c(1164434, 368738)),
                     st_point(c(1203023, 412959)),
                     crs = 3005)

Previously, we could just do this, with sf::st_bbox() embedded in the call:

bcdc_query_geodata("local-and-regional-greenspaces") %>%
  filter(BBOX(st_bbox(two_points, crs = st_crs(two_points))))

## Error: Unable to process query. Did you use a function that should be evaluated locally? If so, try wrapping it in 'local()'.

However you must now use local() to force local evaluation of st_bbox() on your machine in R, before it is translated into a query plan to be executed on the server:

bcdc_query_geodata("local-and-regional-greenspaces") %>%
  filter(BBOX(local(st_bbox(two_points, crs = st_crs(two_points)))))

## Querying 'local-and-regional-greenspaces' record ## • Using collect() on this object will return 1154 features and 19 fields ## • At most six rows of the record are printed here ## ──────────────────────────────────────────────────────────────────────────────────────────────────── ## Simple feature collection with 6 features and 19 fields ## Geometry type: POLYGON ## Dimension: XY ## Bounding box: xmin: 1200113 ymin: 385903.5 xmax: 1202608 ymax: 386561.8 ## Projected CRS: NAD83 / BC Albers ## # A tibble: 6 × 20 ## id LOCAL…¹ PARK_…² PARK_…³ PARK_…⁴ REGIO…⁵ MUNIC…⁶ CIVIC…⁷ CIVIC…⁸ STREE…⁹ LATIT…˟ LONGI…˟ ## <chr> <int> <chr> <chr> <chr> <chr> <chr> <chr> <chr> <chr> <dbl> <dbl> ## 1 WHSE_BASE… 3347 Konuks… Local Green … Capital Distri… <NA> <NA> <NA> 48.5 -123. ## 2 WHSE_BASE… 3304 <NA> Local Trail Capital Distri… <NA> <NA> <NA> 48.5 -123. ## 3 WHSE_BASE… 3380 <NA> Local Water … Capital Distri… <NA> <NA> <NA> 48.5 -123. ## 4 WHSE_BASE… 3369 <NA> Local Water … Capital Distri… <NA> <NA> <NA> 48.5 -123. ## 5 WHSE_BASE… 3453 <NA> Local Water … Capital Distri… <NA> <NA> <NA> 48.5 -123. ## 6 WHSE_BASE… 3361 <NA> Local Trail Capital Distri… <NA> <NA> <NA> 48.5 -123. ## # … with 8 more variables: WHEN_UPDATED <date>, WEBSITE_URL <chr>, LICENCE_COMMENTS <chr>, ## # FEATURE_AREA_SQM <dbl>, FEATURE_LENGTH_M <dbl>, OBJECTID <int>, SE_ANNO_CAD_DATA <chr>, ## # geometry <POLYGON [m]>, and abbreviated variable names ¹LOCAL_REG_GREENSPACE_ID, ²PARK_NAME, ## # ³PARK_TYPE, ⁴PARK_PRIMARY_USE, ⁵REGIONAL_DISTRICT, ⁶MUNICIPALITY, ⁷CIVIC_NUMBER, ## # ⁸CIVIC_NUMBER_SUFFIX, ⁹STREET_NAME, ˟LATITUDE, ˟LONGITUDE