ST_Distance returns 0 instead of 5

[br0ke]

Hey!

ST_Distance function returns incorrect value but _ST_DistanceUncached returns correct. And it's not a rare, weird case: I have two simple rectangles, distance between is 5 meters.

My query (also can be found here ​https://pastebin.com/QTfV19vh):

SELECT
	ST_Distance(f1.geog, f2.geog),
	_ST_DistanceTree(f1.geog, f2.geog),
	_ST_DistanceUncached(f1.geog, f2.geog)
FROM
	LATERAL (
		SELECT ST_Transform(
			ST_SetSRID(
				ST_GeomFromGeoJson('{"type": "Polygon", "coordinates": [[[-5, 0], [-5, 100], [5, 100], [5, 0], [-5, 0]]]}'),
				32641
			),
			4326
		)::geography AS geog
	) AS f1,
	LATERAL (
		SELECT ST_Transform(
			ST_SetSRID(
				ST_GeomFromGeoJson('{"type": "Polygon", "coordinates": [[[-10, 30], [-10, 40], [-20, 40], [-20, 30], [-10, 30]]]}'),
				32641
			),
			4326
		)::geography AS geog
	) AS f2,
	generate_series(1, 10);

Output:

 st_distance | _st_distancetree | _st_distanceuncached 
-------------+------------------+----------------------
           0 |                0 |     4.98653949096524
           0 |                0 |     4.98653949096524
           0 |                0 |     4.98653949096524
           0 |                0 |     4.98653949096524
           0 |                0 |     4.98653949096524
           0 |                0 |     4.98653949096524
           0 |                0 |     4.98653949096524
           0 |                0 |     4.98653949096524
           0 |                0 |     4.98653949096524
           0 |                0 |     4.98653949096524

My PostGIS version:

POSTGIS="2.5.0 r16836" [EXTENSION] PGSQL="100" GEOS="3.5.1-CAPI-1.9.1 r4246" PROJ="Rel. 4.9.3, 15 August 2016" GDAL="GDAL 2.1.2, released 2016/10/24" LIBXML="2.9.4" LIBJSON="0.12.1" LIBPROTOBUF="1.2.1" TOPOLOGY RASTER

My PostgreSQL version:

PostgreSQL 10.5 (Debian 10.5-2.pgdg90+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516, 64-bit

[pramsey]

Good ticket. Can reproduce and appreciate the terse example.

[pramsey]

WITH g AS (
SELECT 
  'POLYGON((58.5112113206308 0,58.5112113200772 0.000901937525203378,58.511300910044 0.000901937636668872,58.5113009105976 0,58.5112113206308 0))'::geography AS a,
  'POLYGON((58.5111665256017 0.000270581240841207,58.5111665255629 0.000360774987788249,58.5110769356128 0.000360774943200728,58.5110769356515 0.000270581207400566,58.5111665256017 0.000270581240841207))'::geography AS b
 )
 SELECT _ST_DistanceTree(a, b), _ST_DistanceUncached(a, b)
 FROM g;

[pramsey]

In 16979:

Patch over bad behaviour for near-parallel boxes and distancetree.
Unfortunately, this "fix" is probably breakable, just put the boxes closer and closer together until we get "close enough to be the same" results from the edge normal comparison again. Only way around this is to do dot product and comparisons in higher-than-double space.
Reworking this example with even closer together inputs would be useful for exposing this problem for eventual complete fix.
Closes #4223

[pramsey]

In 16980:

Patch over bad behaviour for near-parallel boxes and distancetree.
Unfortunately, this "fix" is probably breakable, just put the boxes closer and closer together until we get "close enough to be the same" results from the edge normal comparison again. Only way around this is to do dot product and comparisons in higher-than-double space
References #4223

[pramsey]

In 16981:

Patch over bad behaviour for near-parallel boxes and distancetree.
Unfortunately, this "fix" is probably breakable, just put the boxes closer and closer together until we get "close enough to be the same" results from the edge normal comparison again. Only way around this is to do dot product and comparisons in higher-than-double space
References #4223

[pramsey]

In 16993:

Patch over bad behaviour for near-parallel boxes and distancetree.

Unfortunately, this "fix" is probably breakable, just put the boxes closer and closer together until we get "close enough to be the same" results from the edge normal comparison again. Only way around this is to do dot product and comparisons in higher-than-double space
References #4223