Speaking of PostgreSQL, it’s also worth mentioning PostGIS, which is a fabulous geospatial extension for PostgreSQL. Nelson Minar recommends the book PostGIS in Action to supplement the PostGIS documentation. As an example, PostGIS is improving in the area of doing nearest neighbor searches.
Alternatively, one can try SpatiaLite which embeds GIS capabilities in the self-contained SQLite RDBMS. However, I found SpatiaLite had a lot of moving parts and didn’t get a lot of mileage out of it.
Sideways observation: Pinboard doesn’t make individual item URLs obviously visible in its HTML. A little digging in an RSS feed reveals their existence. Turns out, on the person’s web page, the dates are the permalinks for the bookmarks. Useful for giving credit.