Store GeoJSON in a separate table

I think that even with good indexes, querying the area names from one
table is always going to be slow because there’s so much GeoJSON to scan
past.

This commit splits the data into two tables, one for the names and
grouping IDs and one for the blobs of GeoJSON. So for most pages the app
will never even be looking at the table where the GeoJSON is held.

I don’t know if this is a proper, normalised way of structuring the
data, but it does go brrr.

app/broadcast_areas/repo.py

@@ -37,8 +37,6 @@ class BroadcastAreasRepository(object):
                 name TEXT NOT NULL,
                 broadcast_area_library_id TEXT NOT NULL,
                 broadcast_area_library_group_id TEXT,
-                feature_geojson TEXT NOT NULL,
-                simple_feature_geojson TEXT NOT NULL,
 
                 FOREIGN KEY (broadcast_area_library_id)
                     REFERENCES broadcast_area_libraries(id),
@@ -47,6 +45,13 @@ class BroadcastAreasRepository(object):
                     REFERENCES broadcast_area_library_groups(id)
             )""")
 
+            conn.execute("""
+            CREATE TABLE broadcast_area_features (
+                id TEXT PRIMARY KEY,
+                feature_geojson TEXT NOT NULL,
+                simple_feature_geojson TEXT NOT NULL
+            )""")
+
             conn.execute("""
             CREATE INDEX broadcast_areas_broadcast_area_library_id
             ON broadcast_areas (broadcast_area_library_id);
@@ -69,21 +74,29 @@ class BroadcastAreasRepository(object):
 
     def insert_broadcast_areas(self, areas):
 
-        q = """
+        areas_q = """
         INSERT INTO broadcast_areas (
             id, name,
-            broadcast_area_library_id, broadcast_area_library_group_id,
+            broadcast_area_library_id, broadcast_area_library_group_id
+        )
+        VALUES (?, ?, ?, ?)
+        """
+
+        features_q = """
+        INSERT INTO broadcast_area_features (
+            id,
             feature_geojson, simple_feature_geojson
         )
-        VALUES (?, ?, ?, ?, ?, ?)
+        VALUES (?, ?, ?)
         """
 
         with self.conn() as conn:
             for id, name, area_id, group, feature, simple_feature in areas:
-                conn.execute(q, (
-                    id, name,
-                    area_id, group,
-                    geojson.dumps(feature), geojson.dumps(simple_feature),
+                conn.execute(areas_q, (
+                    id, name, area_id, group,
+                ))
+                conn.execute(features_q, (
+                    id, geojson.dumps(feature), geojson.dumps(simple_feature),
                 ))
 
     def query(self, sql, *args):
@@ -190,7 +203,7 @@ class BroadcastAreasRepository(object):
     def get_feature_for_area(self, area_id):
         q = """
         SELECT feature_geojson
-        FROM broadcast_areas
+        FROM broadcast_area_features
         WHERE id = ?
         """
 
@@ -201,7 +214,7 @@ class BroadcastAreasRepository(object):
     def get_simple_feature_for_area(self, area_id):
         q = """
         SELECT simple_feature_geojson
-        FROM broadcast_areas
+        FROM broadcast_area_features
         WHERE id = ?
         """