questions about RECTANGLE_INTERSECTION flag

[arno]

Hi,

Long story short:
Enabling USE_RECTANGLE_INTERSECTION flag speeds up postgis ST_Intersection by 60x for my use case

Long story long:
I recently wrote a strava like heatmap. It requires computing the intersections of my gpx tracks with tile layers. The PostGIS query used is:

```
const query = `SELECT ST_ASGeoJson(ST_Intersection(geom, ST_MakeEnvelope($1, $2, $3, $4, 4326))) AS geom from ${this.#tableName}`
```

It is the performance bottleneck on my application. I have about 2000 gpx tracks. On a large tile, where 1300 geometries intersect, the query takes about 2500ms to run. I tried various query optimizations without sucess.

But if I rebuild libgeos with USE_RECTANGLE_INTERSECTION flag set, the query succeeds in about 40ms. (ie: 60x speedup!) (https://github.com/libgeos/geos/blob/main/src/geom/Geometry.cpp#L76)

Is there a way this optimization could be enabled in libgeos?

I understand there are differences between the current intersection operation and the rectangle intersection optimization:

+ If the geometries don't intersect, the result is an empty geometry of type geometryA. With RectangleIntersection, it is an empty GEOMETRYCOLLECTION
```
% geosop -a 'LINESTRING(10 10, 20 20)' -b 'POLYGON((110 10, 120 10, 120 20, 110 20, 110 10))' clipRect
GEOMETRYCOLLECTION EMPTY
% geosop -a 'LINESTRING(10 10, 20 20)' -b 'POLYGON((110 10, 120 10, 120 20, 110 20, 110 10))' intersection
LINESTRING EMPTY
```

But hopefully, it shouldn't be too hard to add a special mode to RectangleIntersection to return the expected geometry.

+ The resulting geometries gets sometimes rearranged differently. See for example unit test 4 of capi::GEOSIntersection. 2 linestrings are expected. They would not appear with rectangle intersection. By the way, I am pretty novice with geometry computation, and I don't really understand why the linestrings are expected in that case.

Another example: where a linestring self-intersects, and the result will be split at the intersection point. For some reason, it does not seem to happen when the linestring is fully within the rectangle boundary (I'm also confused why that is the case).

```
% geosop -a 'LINESTRING(13 12, 50 50, 50 15, 10 15)' -b 'POLYGON((10 10, 20 10, 20 20, 10 20, 10 10))' intersection
MULTILINESTRING ((13 12, 15.921052631578947 15), (15.921052631578947 15, 20 19.18918918918919), (20 15, 15.921052631578947 15), (15.921052631578947 15, 10 15))
% geosop -a 'LINESTRING(13 12, 50 50, 50 15, 10 15)' -b 'POLYGON((10 10, 20 10, 20 20, 10 20, 10 10))' clipRect
MULTILINESTRING ((13 12, 20 19.189189189189186), (20 15, 10 15))
```

Are there other differences between the current intersection operation, and the rectangle intersection operation? Or any other issue or reasons why that optimization is not enabled?

Thank you

Arno

PS: I attach the patch I used to enable Rectangle Intersection
-------------- next part --------------
diff --git SRC src/geom/Geometry.cpp DST src/geom/Geometry.cpp
index 433933587..8f960c0fd 100644
--- SRC src/geom/Geometry.cpp	
+++ DST src/geom/Geometry.cpp	
@@ -73,7 +73,7 @@
 #endif
 
 #define SHORTCIRCUIT_PREDICATES 1
-//#define USE_RECTANGLE_INTERSECTION 1
+#define USE_RECTANGLE_INTERSECTION 1
 #define USE_RELATENG 1
 
 using namespace geos::algorithm;
@@ -603,13 +603,13 @@ Geometry::intersection(const Geometry* other) const
         const Envelope* env = getEnvelopeInternal();
         Rectangle rect(env->getMinX(), env->getMinY(),
                        env->getMaxX(), env->getMaxY());
-        return RectangleIntersection::clip(*other, rect).release();
+        return RectangleIntersection::clip(*other, rect);
     }
     if(other->isRectangle()) {
         const Envelope* env = other->getEnvelopeInternal();
         Rectangle rect(env->getMinX(), env->getMinY(),
                        env->getMaxX(), env->getMaxY());
-        return RectangleIntersection::clip(*this, rect).release();
+        return RectangleIntersection::clip(*this, rect);
     }
 #endif