# set connection
conn <- dbConnect("PostgreSQL",user="postgres",password="pass",port=5432,dbname="postgis_user")

# set query and reproject to EPSG:4326 for GeoJSON creation
# (the real query is more involved with a bounding box etc)
qry <- "SELECT ST_Transform(table.geom, 4326) as geom FROM table"

# i then read as an sf object
pols <- st_read_db(conn, query=qry, geom="geom")

# and convert to GeoJSON
pols.js <- geojsonio::geojson_json(pols)

qry1 <- "SELECT ST_AsGeoJSON(ST_Transform(geom,4326)) FROM table"
pols1 <- dbGetQuery(conn,qry1)
## no error messages but no display in Leaflet (incorrectly constructed geoJSON i assume, returned as a data.frame)

qry2 <- "SELECT json_build_object(
'type', 'FeatureCollection',

'features', json_agg(
    json_build_object(
        'type',       'Feature',
        'id',         gid,
        'geometry',   ST_AsGeoJSON(ST_ForceRHR(st_transform(geom,4326)))::json,
    'properties', jsonb_set(row_to_json(table)::jsonb,'{geom}','0',false)
)
)
)
FROM table"
    pols2 <- dbGetQuery(conn,qry2)
    ## hangs and errors: "DETAIL:  Cannot enlarge string buffer containing 1073740688 bytes by 3167 more bytes."

qry3 <- "SELECT row_to_json(f) As feature
  FROM (SELECT 'Feature' As type,
                ST_AsGeoJSON(ST_Transform(l.geom, 4326))::json As geometry,
                row_to_json((SELECT l FROM (SELECT id, 'l' AS type) As l))
As properties
  FROM table As l) As f"
pols3 <- dbGetQuery(conn,qry3)
## this gives a similar output to attempt 1 above but gives a warning "RS-DBI driver
## warning: (unrecognized PostgreSQL field type json (id:114) in column 0)", which
## i'm guessing is bcos i've got the syntax wrong.