The Notions of Consistency and Predicate Locks in a Database System
In database systems, users access shared data
under the assumption that the data satisfies 
certain consistency constraints.  This paper defines the
concepts of transaction, consistency and schedule 
and shows that consistency requires that a transaction
cannot request new locks after releasing a lock. 
 Then it is argued that a transaction needs to lock a logical
rather than a physical subset of the database. 
 These subsets may be specified by predicates.  An
implementation of predicate locks which satisfies 
the consistency condition is suggested.
CACM November, 1976
Eswarran, K..
Gray, J..
Lorie, R..
Traiger, I. L.
