The approach shown in the article used an analytical function to filter all immediate children of a node in a recursive CTE.
This allowed us to filter a node’s children on the level more efficiently than R-Tree or B-Tree approaches do (since they rely on COUNT(*)).
That solution was pure SQL and it was quite fast, but not optimal.

Mas:

http://explainextended.com/2010/03/02/postgresql-using-recursive-functions-in-nested-sets/