You should probably look at the view definition for acs_object_party_privilege_map. It might just be that the second query represents the view. In general it is better to replace well used joins with a view so they are easier to read and reuse, and so that you avoid confusion and/or bugs. In postgres, views are still handled like the original def for query planning, but maybe the setup is faster, which would be another benefit.
If there is a pl function which does a permission_p check, this should be optimized and then used by the Tcl proc directly. However, if you are looking for a permission as part of a where clause, the view (map) should be used for the reasons Dave gave above, this allows the query planner/optimizer to speed things up.
