Polygon/multipolygon columns: incorrect write, correct read

[pachadotdev]

Incorrect write

If we open a regular DBI connection, as in the next example, we won't correctly write to Postgres with Postgis correctly configured.

library(RPostgres)
library(sf)
library(archive)

con <- dbConnect(
  drv = Postgres(),
  dbname = "shiny"
)

download.file("https://www2.census.gov/geo/tiger/TIGER2014/TRACT/tl_2014_01_tract.zip", "tl_2014_01_tract.zip")

archive_extract("tl_2014_01_tract.zip")

d <- read_sf("tl_2014_01_tract.shp")

# this saves geometry as text
# sf::dbWriteTable(con, "carto", d, append = TRUE)
# RPostgres::dbWriteTable(con, "carto", d, append = TRUE)

# this saves geometry as geometry (not working)
# st_write(d, con, "carto", append = TRUE, layer = "carto", layer_options = c("GEOMETRY_NAME=geometry"))

# this saves geometry as geometry
st_write(
  d2,
  "PG:host=localhost port=5432 dbname=shiny user=xxx password=yyy",
  layer = "carto",
  layer_options = c("GEOMETRY_NAME=geometry"),
  append = TRUE
)

Correct read

library(ggplot2)
library(dplyr)

# Ggplot2 won't like the <pq_gmtry> but it's correct
d2 <- tbl(con, "carto") %>%
    filter(countyfp == "001") %>%
    collect()

glimpse(d2)

... TRUNCATED OUTPUT ...
$ countyfp <chr> "001", "001", "001", "001", "001", "001", "001", "001", "001"…
$ tractce  <chr> "021100", "021000", "020900", "020100", "020300", "020400", "…
$ aland    <dbl> 478168607, 386854992, 292754413, 9810181, 5349273, 6382705, 1…
$ geometry <pq_gmtry> 0106000020AD100000010000000103000000010000003B0A0000A437…
...

# convert pq_gmtry to multipolygon
d2 <- d2 %>%
  mutate(geometry = st_as_sfc(geometry))

# plot ok
ggplot() +
    geom_sf(data = d2, aes(fill = aland, geometry = geometry))

dbDisconnect(con)

Here's my session info:

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[dpprdan]

This only concerns the sf::dbWriteTable() method, so I think this is better handled there than here.

However, this is partly caused by RPostgres::dbWriteTable() stop()ing if append && !is.null(field.types)

RPostgres/R/dbWriteTable_PqConnection_character_data.frame.R

Lines 24 to 26 in 58a052b

	if (append && !is.null(field.types)) {
	stopc("Cannot specify `field.types` with `append = TRUE`")
	}

This also happens when the table does not (yet) exist, i.e. when there is nothing to append to.

Do you think it would be possible to move this check a few lines down and only stop if the table is found, @krlmlr?

RPostgres/R/dbWriteTable_PqConnection_character_data.frame.R

Line 47 in 58a052b

found <- dbExistsTable(conn, name)

Right now, dbWriteTable will create the table anyway if no field.types are provided.

RPostgres/R/dbWriteTable_PqConnection_character_data.frame.R

Lines 60 to 63 in 58a052b

	if (!found || overwrite) {
	if (is.null(field.types)) {
	combined_field_types <- lapply(value, dbDataType, dbObj = conn)
	} else {

The downside is that dbWriteTable(append = TRUE, field.types = "something") will work when the table does not exist but stop when it does. This might be confusing. However, we could ameliorate this by issuing a message "You wanted to append, but the table does not exist yet so we created it".

Of course, third-party methods have to provide custom field.types for their datatypes, like sfc, so field.types cannot be NULL for them. So they cannot create a table currently if append = TRUE but the table does not exist.

[krlmlr]

Thanks, missed that. IIUC, we can fix this by fixing sf::dbWriteTable()?

[dpprdan]

Thanks, missed that. IIUC, we can fix this by fixing sf::dbWriteTable()?

Yes. But the fix is a bit ugly (IMO, i.e. overriding function arguments).

IMHO it would be best, i.e. cleanest, if append were only possible if the table already exists, but that's probably too much of a breaking change.

[krlmlr]

There are new generics dbCreateTable() and dbAppendTable(), does sf support these?

[dpprdan]

Ha, I just had a similar thought, i.e. refactor sf::dbWriteTable() using dbCreateTable() and dbAppendTable().

So no, sf does not support/use them yet.

Anyway, I am not convinced by my "create table if append = TRUE" proposal above myself anymore, so this can be closed IMO.

[dpprdan]

Fixed in {sf}: r-spatial/sf#2223

[krlmlr]

Closing, fixed downstream. Thanks @dpprdan!