Databases Reference
In-Depth Information
Figure 8-6. Road segments
Our app analyzes each road segment as a data tuple, calculating a center point for each.
We use a geohash value to construct a bounding box around that center point, then join
the data to collect metrics for all the trees nearby, as
Figure 8-7
shows.
Figure 8-7. Trees near road segments
The join occurs in the
get-shade
function where both the roads and trees tuples ref‐
erence the
?geohash
field:
(
defn
tree-distance
[
tree_lat
tree_lng
road_lat
road_lng
]
"calculates distance from a tree to the midpoint of a road segment"
(
let
[
y
(
-
tree_lat
road_lat
)
x
(
-
tree_lng
road_lng
)]
(
Math/sqrt
(
+
(
Math/pow
y
2.0
)
(
Math/pow
x
2.0
)))))
(
defn
get-shade
[
trees
roads
]
"subquery to join the tree and road estimates, to maximize for shade"
(
<-
[
?road_name
?geohash
?road_lat
?road_lng
?road_alt
