Abstract: The present invention relates to a system and methods for fine-granularity concurrency control in a parallel database. Very fine granularity (at the level of B-tree records) is implemented in a B-tree. Our method applies to B-trees, B-trees with variable keys, and their applications, such as semantic and object-oriented databases. Our method involves accumulating a transaction and then "optimistically" validating it, while attaining high efficiency with maximum semantic safety. "Lazy queries"--an efficient method for finding the intersection of two large queries--is provided for the system.