Importing Simple Feature data into topology

PostGIS can convert Simple Feature geometries into topology. Let's assume a countries table containing country boundaries from the Natural Earth dataset, with a geometry column named geom, and the WGS84 coordinate system.

Checking the validity of input geometries

The first step is to check the validity of input geometries. PostGIS will refuse to convert invalid geometries to topology elements, so it's very important to fix them or filter them out. We will use the ST_IsValidReason and ST_IsValid functions that we learned in Chapter 1, Importing Spatial Data:

SELECT ST_IsValidReason(geom) FROM countries WHERE ST_IsValid(geom) = FALSE; 

Luckily, in this case the query returned 0 rows, meaning that all geometries...