Improve performance of /nearest.

The nearest postcode lookup performance reduced dramatically when tried
with Postgres 9.6 and PostGIS 2.3. This commit updates the query to use
the ‘<->’ centroid distance operator instead, which makes better use of
the geoindex and is much faster to execute.

Handily, the work @dracos did to introduce the TrigramDistance function
into Django 1.10 was easily adaptable for this purpose, as the operator
used is the same.

mapit/views/postcodes.py

@@ -4,9 +4,11 @@
 
 from django.shortcuts import redirect, render
 from django.contrib.gis.geos import Point
-from django.contrib.gis.measure import D
 from django.contrib.gis.db.models import Collect
 from django.views.decorators.csrf import csrf_exempt
+from django.db.models import FloatField
+from django.db.models.expressions import Func, Value
+from django.contrib.gis.db.backends.postgis.adapter import PostGISAdapter
 
 from mapit.models import Postcode, Area, Generation
 from mapit.utils import is_valid_postcode, is_valid_partial_postcode
@@ -36,6 +38,17 @@
 }
 
 
+class GeometryCentroidDistance(Func):
+    function = ''
+    arg_joiner = ' <-> '
+
+    def __init__(self, expression, string, **extra):
+        print(expression, string, extra)
+        if not hasattr(string, 'resolve_expression'):
+            string = Value(string)
+        super(GeometryCentroidDistance, self).__init__(expression, string, output_field=FloatField(), **extra)
+
+
 @ratelimit(minutes=3, requests=100)
 def postcode(request, postcode, format=None):
     if hasattr(countries, 'canonical_postcode'):
@@ -156,8 +169,9 @@ def nearest(request, srid, x, y, format='json'):
     location = Point(float(x), float(y), srid=int(srid))
     set_timeout(format)
     try:
-        postcode = Postcode.objects.filter(
-            location__distance_gte=(location, D(mi=0))).distance(location).order_by('distance')[0]
+        postcode = Postcode.objects.annotate(
+            centroid_distance=GeometryCentroidDistance('location', PostGISAdapter(location))
+            ).filter(centroid_distance__gte=0).distance(location).order_by('centroid_distance')[0]
     except DatabaseError as e:
         if 'Cannot find SRID' in e.args[0]:
             raise ViewException(format, e.args[0], 400)