This may be another issue of the problem discussed here: http://postgresql.1045698.n5.nabble.com/avoid-prepared-statements-on-complex-queries-td4996363.html (Kris Jurka explains the crux of it in that thread). Note that it seems the preparing/planning interaction was not the poster's actual problem, but it may have been yours. As Tom Lane notes in that thread, this should get better in 9.2.
pgsql-performance by date:
Соглашаюсь с условиями обработки персональных данных
