Comment 2 for bug 132485

Another thing to consider is what behaviour should a connection or store exhibit between the two phases of commit. The two options I can see are:
 1. disallow all queries besides the final commit or rollback
 2. consider prepare() to mark the start of the next transaction (this is what Postgres effectively allows, allowing you to accumulate multiple prepared transactions).

I'd probably lean towards (1) due to it being simpler and more likely to fit other databases. This probably requires more investigation though.