JPA2 with the Criteria API seems to generate invalid SQL for PostgreSQL. For this code:

Root<DBObjectAccessCounter> from = query.from(DBObjectAccessCounter.class);
Path<DBObject> object = from.get(DBObjectAccessCounter_.object);
Expression<Long> sum = builder.sumAsLong(from.get(DBObjectAccessCounter_.count));
query.multiselect(object, sum).groupBy(object);

I get the following exception:

ERROR: column "dbobject1_.id" must appear in the GROUP BY
clause or be used in an aggregate function

The generated SQL is

select dbobjectac0_.object_id as col_0_0_,
sum(dbobjectac0_.count) as col_1_0_, dbobject1_.id as id1001_,
dbobject1_.name as name1013_,
dbobject1_.lastChanged as lastChan2_1013_,
dbobject1_.type_id as type3_1013_
from DBObjectAccessCounter dbobjectac0_
inner join DBObject dbobject1_
on dbobjectac0_.object_id=dbobject1_.id
group by dbobjectac0_.object_id

This is also the case for a simplified version:

Root<DBObjectAccessCounter> from = query.from(DBObjectAccessCounter.class);
Path<DBObject> object = from.get(DBObjectAccessCounter_.object);
query.select(object).groupBy(object);

Here the following SQL is generated:

select dbobject1_.id as id924_, dbobject1_.name as name933_,
dbobject1_.lastChanged as lastChan2_933_,
dbobject1_.type_id as type3_933_
from DBObjectAccessCounter dbobjectac0_
inner join DBObject dbobject1_
on dbobjectac0_.object_id=dbobject1_.id
group by dbobjectac0_.object_id

This issue is also discussed on Stackoverflow: http://stackoverflow.com/questions/12403399/

I attached a test case for all three examples (multiselect, simplified, simplified with group by id)