| 摘要 |
A database management system (DBMS) is modified to provide improved concurrent usage of database objects, particularly when the system is executing long lived transactions. A subset of the transactions access database objects using parameterized read and parameterized write access modes. Each transaction using a parameterized write mode of access for a database objet specifies a write access mode, and a write access mode parameter, where the parameter indicates a data reliability classification. Each transaction using a parameterized read mode of access for a database object specifies a read access mode, and a read access mode parameter, where the parameter indicates one or more reliability classifications that are acceptable to the transaction. Whenever a transaction requests access to a specified database object, the DBMS generates a corresponding lock request for the object. If the lock request is parameterized lock request, a corresponding parameterized lock request is generated. A lock manager processes each lock request by checking to see if any outstanding, previously granted lock is a unconditionally conflicting or conditionally conflicting wit the requested lock. A query executer queries requesting access to information stored in the database (or other data processing tasks). The query executer includes annotated nullable logic for evaluating expressions containing at least one annotated value as an operand. The annotated nullable logic includes logic for combining annotated values, for comparing annotated values so as to generate annotated truth values, and for combining annotated truth values in accordance with a predefined set of rules.
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