In my quest to understand spatial GIS functionality, I have come to the ultimate goal: evaluation the actual database products themselves.
PostgreSQL / PostGIS
PostGIS is a variant of PostgreSQL with spatial extensions. The main reason for maintaining the GIS feature set outside of PostgreSQL proper seems to be licensing: the spatial extensions are LGPL GPL licensed.
PostGIS is widely recognized as the most mature and feature-rich GIS implementation for SQL databases (and perhaps any database), matched only by the costly Spatial extension for the Oracle Enterprise database. (See comparisons in the links below.)