PostGIS ST_Intersects does not work as expected - it returns false although ST_Covers return true

Question

I have two polygons such that polygon A (pie-shaped) covers polygon B (rectangle) as shown here: https://wktmap.com/?6aaf8c11.

Why does PostGIS return false for ST_Intersects(A, B), but true for ST_Covers(A, B)? It does not make any sense.

SELECT st_intersects(ST_GeomFromText('POLYGON((16.00061 45.822171,15.99285240563651 45.8293452320342,15.990050859670909 45.82732350122485,15.988282915930574 45.82479740769603,15.987721633066801 45.82201422308267,15.988421953356983 45.8192463848863,16.00061 45.822171))',4326), ST_GeomFromText('POLYGON((15.989 45.82, 15.989 45.82, 15.989 45.82, 15.989 45.82, 15.989 45.82))',4326));
 st_intersects 
---------------
 f
(1 row)

SELECT st_covers(ST_GeomFromText('POLYGON((16.00061 45.822171,15.99285240563651 45.8293452320342,15.990050859670909 45.82732350122485,15.988282915930574 45.82479740769603,15.987721633066801 45.82201422308267,15.988421953356983 45.8192463848863,16.00061 45.822171))',4326), ST_GeomFromText('POLYGON((15.989 45.82, 15.989 45.82, 15.989 45.82, 15.989 45.82, 15.989 45.82))',4326));
 st_covers 
-----------
 t
(1 row)

SELECT PostGIS_version();
            postgis_version            
---------------------------------------
 3.2 USE_GEOS=1 USE_PROJ=1 USE_STATS=1
(1 row)

SELECT version();
                                                                version                                                                 
----------------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 14.12 (Ubuntu 14.12-0ubuntu0.22.04.1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0, 64-bit
(1 row)

Answer 1

The small rectangle is an invalid polygon, as all coordinates are identical:

ST_GeomFromText('POLYGON((15.989 45.82, 15.989 45.82, 15.989 45.82, 15.989 45.82, 15.989 45.82))',4326))

Calculating spatial relations with invalid polygons leads to undefined result...

Answer 2

One possible reason is definitions of terms.

I would expect a function called "Intersects" to look at whether the boundaries of two shapes cross each other, because that's what "Intersect" means. Looking at your Wktmap link, those two shapes do not intersect because the boundaries do not cross each other.

Disclaimer: I know nothing about PostGIS, this is a geometry-based answer.