Parcel Distance to Hydrant

p2h_distance.sql

-- An example of a lateral join driving a nearest-neighbor 
-- distance calculation
SELECT
  parcels.*,
-- keep the hydrant id around, might be useful later  
  hydrants.cartodb_id as hydrant_cartodb_id,
-- calculate distance over the spheroid using geography distance
  ST_Distance(geography(hydrants.the_geom), geography(parcels.the_geom)) as distance
FROM 
-- for this data, removing the duplicate parcel geometries and
-- null attributed parcels cuts the size by 2/3 so a good
-- performance improvement to filter records here
  (SELECT DISTINCT ON (the_geom) *
   FROM parcelsshp 
   WHERE pid IS NOT NULL) AS parcels 
-- cross join takes every combination of join records
-- without restriction, which is fine since we're limiting
-- the hydrants side of the join to just one return record
-- per parcel
CROSS JOIN LATERAL
-- the guts of the query, actually do a KNN query for each
-- candidate parcel on the other side of the join
  (SELECT cartodb_id, the_geom
   FROM hydrantsshp
   ORDER BY parcels.the_geom_webmercator <-> the_geom_webmercator 
   LIMIT 1) AS hydrants