When using INNER join and a join condition that is always true, the RelToSqlConverter may generate an invalid SQL.
which produces error (on Postgres):
The CROSS JOIN syntax was introduced in SQL 92 and seems to be widely supported by DB engines, therefore I suggest to change the default in SqlDialect#emulateJoinTypeForCrossJoin from JoinType.COMMA to JoinType.CROSS.
I have checked all supported DB products from SqlDialect.DatabaseProduct and they all seem to support the CROSS JOIN syntax, except these, which I have not been able to verify: Infobright, Luciddb, Paraccel, Netezza. For sake of backward compatibility, I suggest to override the emulateJoinTypeForCrossJoin method in these dialects to behave as before, i.e. JoinType.COMMA.