Point to Polygon nearest distance DS_distance is not using geography index & knn <-> or <#> does not give result in order

PostGIS Point-to-Polygon Distances: Why Your Nearest Neighbor Might Not Be Who You Think

PostGIS is a fantastic tool for playing with geographic data. But let’s be honest, sometimes getting it to do what you think it should do can be a bit… frustrating. Ever tried finding the closest polygon to a point, only to find your query taking forever, or worse, returning seemingly random results? You’re not alone!

One head-scratcher I’ve run into (and I bet you have too) is calculating the distance between a point and a polygon. You’d think, “easy peasy, spatial index, done!” But sometimes, DS_distance (or its ST_Distance equivalent when you’re using geography types) just seems to ignore your index and decides to scan the entire table. Talk about slow!

Why does this happen? Well, under the hood, PostGIS has to guess the best way to run your query. It’s like a GPS trying to find the fastest route – sometimes it makes a weird choice. The query planner estimates the cost of using the index, and if it thinks a full table scan is cheaper (even when it isn’t!), that’s what you get. This can depend on how many rows it thinks will be returned, how big your dataset is, and the statistics it has about your data.

So, what can you do about it? First, double-check you actually have a GiST index on your geography column. That’s the foundation. Then, give PostGIS a little help by running ANALYZE on your table. This updates the planner’s statistics and helps it make smarter decisions. Think of it as giving your GPS a map update.

Sometimes, you need to be a bit more forceful. Try rewriting your query to nudge the planner in the right direction. A bounding box operator (&&) can be your friend here. Pre-filter your data to only consider polygons within a certain distance of your point’s bounding box before calculating the exact distance. It’s like saying, “Hey PostGIS, focus on this area first!”

But wait, there’s more! Let’s talk about KNN queries – finding the K-Nearest Neighbors. You use operators like <-> (for geometry) or <#> (for geography), expecting results to pop out in order of distance, right? Well, sometimes the order seems… off. I’ve definitely seen cases where the supposed “nearest” neighbor was clearly further away than others in the list.

This usually boils down to how PostGIS approximates distances on the Earth’s surface (when using geography types). These approximations are generally good, but they can introduce tiny errors that mess with the ordering, especially when you’re dealing with large distances or points that are very close together.

The fix? Don’t rely solely on the KNN operator for ordering. Add an explicit ORDER BY clause using ST_Distance (or ST_DistanceSpheroid for extra accuracy). This forces PostGIS to sort the results based on the actual calculated distance, overriding any potential quirks in the KNN operator’s internal ordering.

Instead of just ORDER BY location <#> ‘SRID=4326;POINT(-71.06 42.35)’, use ORDER BY ST_Distance(location, ‘SRID=4326;POINT(-71.06 42.35)’). Trust me, it’s worth the extra typing for the peace of mind.

Bottom line? PostGIS is powerful, but it’s not magic. Getting the most out of it requires understanding its quirks and knowing how to guide it along. By paying attention to indexing, query planning, and ordering, you can avoid common pitfalls and unlock the true potential of your spatial data. Happy querying!