Postgres returning incorrect results with distance join queries

Question

I created a table containing locations along with their spatial coordinates expressed in (latitude, longitude (in degrees)) in postgres. Commands which I used for the same are:

create table spatialTest(name character varying(100), the_geo geography);

\copy spatialTest(name,the_geo) FROM 'testSpatial.csv' DELIMITERS E'\t' CSV HEADER;

I created a table containing locations along with their spatial coordinates expressed in (latitude, longitude (in degrees)) in postgres. Commands which I used for the same are:

create table spatialTest(name character varying(100), the_geo geography);

\copy spatialTest(name,the_geo) FROM 'testSpatial.csv' DELIMITERS E'\t' CSV HEADER;

testSpatial.csv contains the following values:

A   SRID=4326;POLYGON((0.178773,-127.681841|0.178711,-127.681962|0.179125,-127.682083|0.179176,-127.682006|0.179153,-127.681986|0.179143,-127.681962|0.179147,-127.681935|0.179166,-127.681913|0.179195,-127.681897|0.179244,-127.681886|0.179284,-127.681887|0.179336,-127.681904|0.179464,-127.681757|0.179489,-127.681736|0.179429,-127.681680|0.179370,-127.681516|0.179221,-127.681331|0.179184,-127.681185|0.179051,-127.681264|0.178822,-127.681499|0.178761,-127.681698|0.178796,-127.681703|0.178839,-127.681721|0.178857,-127.681736|0.178861,-127.681740|0.178871,-127.681756|0.178873,-127.681782|0.178859,-127.681809|0.178843,-127.681825|0.178812,-127.681839|0.178773,-127.681841))
B   SRID=4326;POINT(0.628912,-127.700922)

Now I want to find all spatial locations which are within a distance of 50 km of each other. For doing so I used the following command:

select s1.name, s2.name  from spatialTest s1, 
     spatialTest s2  where ST_DWithin(s1.the_geo, s2.the_geo, 50000);

However, to my surprise I found that although A and B are separated from each other by a distance greater than 50 km (50.0995 km to be precise. Found using Chris Veness's geodesy formulae​ (Calculate distance between a point and a line segment in latitude and longitude)), yet they are returned by postgres as results.

Where am I going wrong?

I am using PostgreSQL 9.6devel and Postgis version which I am using is: POSTGIS="2.2.1 r14555"

Answer 1

SRID 4326 is measured in degrees, though you can cast it to geography and let ST_DWithin utilize meters:

SELECT s1.name, s2.name 
FROM spatialTest s1, spatialTest s2
WHERE ST_DWithin(s1.the_geo::geography, s2.the_geo::geography, 50000);