1. A method comprising: establishing multiple sessions with a database system, each session associated with at least one transaction; identifying transactions that operate on the same set of one or more tuples; re-allocating transactions between or among the sessions such that the identified transactions that operate on the same set of one or more tuples are allocated to one of the sessions; identifying statements in a particular one of the transactions that specify modification operations that are commutative and associative; combining the identified statements into one statement; and submitting the one statement to the database system.