It look like we need to fix HSQL, but I don't know how badly broken HSQL is.
If I originally define the column with "not null" and then alter the column but I don't (since I can't) say "not null" does HSQL continue to treat it as "not null" or is it now nullable?
The former seems non-intuitive.
The latter implies that no altered column can ever have a non null constraint, which for VoltDB implies that no altered column can be used as a partition key (or that no partition key can ever be altered). This is sad.