openacs 5.7 does NOT work with pg 9+ (see e.g. http://www.openacs.org/xowiki/openacs-compatibility-matrix); also http://www.openacs.org/xowiki/Postgres_9 mentions explicitly openacs 5.8.
We should bundle a release of openacs 5.8 rather soon now to avoid more confusions.
all the best -gustaf neumann