Update 'Dump OSM data to a postgis database'

Dump-OSM-data-to-a-postgis-database.md

@@ -92,4 +92,21 @@ wget https://download.openstreetmap.fr/extracts/north-america/mexico-latest.osm.
 sudo -u postgres osm2pgsql --create --database pgsnapshot --cache 20000 --number-processes 2 --slim -P 5435 --keep-coastlines --extra-attributes --hstore-all mexico-latest.osm.pbf
 ```
 
-Then change the postgresql.conf and pg_hba.conf files to allow remote connections to the database.
+Then change the postgresql.conf and pg_hba.conf files to allow remote connections to the database.
+
+## Interesting Postgis features
+
+You can compute minimum distances with KNN as follows:
+
+```
+select
+	st_distance(st_transform(st_setsrid(st_point(-120, 12), 4326), 3857), way) as distance
+from
+	public.planet_osm_roads
+where
+	highway in ('motorway', 'trunk', 'primary', 'secondary')
+order by
+	st_transform(st_setsrid(st_point(-120, 12), 4326), 3857) <-> way
+limit
+	1
+```