It seems that some of the constraints, primary/foreign keys, unique fields etc generate too big indexes for Firebird.
Here's a tool that calculates wether or not index sizes will overflow.
I hope that future LDAP support will obsolete user/role related table's, but there will still be tables that are used by the portal. CMS portlet brings alone half of the tables, but for the core functionality there are just couple of tables left. Could database requirement could be made completely optional?
By reconsidering existing table normalization (removing lexigographical keys) and the actual need for some of the constraints (uniqueness), it still might be possible to make portal run against Firebird and many more db's with working Hibernate dialect, but having legacy restrictions.
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Please note when reproducing the problem that Hibernate doesn't automatically detect Firebird so please make sure to eiher specify the dialect by setting the system property 'hibernate.dialect' as net.sf.hibernate.dialect.FirebirdDialect or manyually change every single hibernate config file under the portal to have dialect element with similar value as above.