There are a number of reasons why an invalid geometry could result in your database; for example, rings composed of polygons must be closed and cannot self-intersect or share more than one point with another ring.
After importing the country shapefile using the ST_IsValid and ST_IsValidReason functions, you will have figured out that four of the imported geometries are invalid, all because their polygons have self-intersecting rings.
At this point, a good way to investigate the invalid multipolygon geometry is by decomposing the polygon in to its component rings and checking out the invalid ones. For this purpose, we have exported the geometry of the ring causing the invalidity, using the ST_GeometryN function, which is able ...
