COUNT doesn't work for Polymorphism

jdo.test.Plan extends from jdo.test.ShareItem, when executing:
(1) SELECT p FROM jdo.test.Plan p WHERE p.state = $1
everything is good. But when try this:
(2) SELECT count(p.id) FROM jdo.test.Plan p WHERE p.state = $1
the exception will be thrown out:
java.sql.SQLException: Unknown table 'shareitem' in where clause

Statement (1) is excuted by SQLEngine, while (2) by SimpleQueryExecutor. SimpleQueryExecutor DOES resolve and field correctly, but forgets to join the parent table.