One of the new features in PostgreSQL 8.4 was the “WITH RECURSIVE” clause available for queries. It allows you to define a subquery based on a recursive term — fancy language for a function that calls itself. One of the favorite uses of recursion is walking a network. Geospatial applications use networks all the time: electrical grids, stream systems, and storm sewers are all directed networks (they have unidirectional flow).
Here’s an example of network walking using a simple collection of segments. As is common in many GIS applications, the segment are implicitly connected — their end points are coincident with the start points of other segments.

Mas:

http://blog.opengeo.org/2010/07/21/network-walking-in-postgis/