Composing and decomposing polygons

Polygons in PostGIS are made of rings. A ring is a closed LineString-that is, the first and last point have the same coordinates. A polygon must have only one outline (also called a shell, or exterior ring) and can have zero or more interior rings (holes), which must be contained within the outer ring. The winding order of vertices is irrelevant (in contrast to some other GIS formats, most notably the Shapefile format). This means that if a polygon feature has more than one exterior ring, it must be saved as a MultiPolygon.