INCORPORATING TIME IN OBJECT ORIENTED DATABASES USING ATTRIBUTE VERSIONING.

     DRAWBACK  OF  TUPLE  VERSIONING

Whenever one attribute value is changed, a whole new tuple version is created, even though all the other attribute values will be identical to the previous tuple version.

    ATTRIBUTE VERSIONING.

*              Database systems that support complex structured objects, such as object databases  or object-relational systems .This approach is called attribute versioning.

*              Each attribute that changes over time is called a time-varying attribute.

*              Attributes that do not change are called non-time-varying and are not associated with the temporal periods.

*              The temporal periods may represent valid time, transaction time, or Bitemporal, depending on the application requirements.

EXAMPLE

Employee(Name,SSN,Salary,DNo,SupervisorSsn)

Dept(DName,DNo,TotalSalary,ManagerSsn)

Non-time-varying attributes (they do not change over time).

Name and  social security number

           

Time-varying attributes (they may change over time).

salary, department, and supervisor

 Each time-varying attribute is represented as a list of tuples

       <valid_start_time, valid_end_time, value>, ordered by valid start time.

For Bitemporal Databases

     <valid_start_time, valid_end_time, trans_start_time, trans_end_time, value>

Temporal Valid time for  EMPLOYEE_VT Object Class

Class TEMPORAL_SALARY

{ attribute              Date                   Valid_start_time;

   attribute              Date                    Valid_end_time;

   attribute              float                    Salary;

};

Class TEMPORAL_LIFESPAN

{ attribute              Date                             Valid_start_time;

   attribute              Date                             Valid_end_time;

 };

Class EMPLOYEE_VT

{   extent EMPLOYEES    }

{attribute                list<TEMPORAL_LIFESPAN>                     lifespan;

 attribute                String                                                      Name;

attribute                 String                                                      SSN;

attribute                 list<TEMPORAL_SALARY>                        Sal_history;

Whenever an attribute is changed in this model, the current attribute version is closed and a new attribute version for this attribute only is appended to the list.

  Lifespan temporal attribute :

1.A lifespan temporal attribute associated  with the whole object indicates the valid time of existence for the whole object.

2.Logical deletion of the object is implemented by closing the   lifespan.

3. Constraint : The  time period of an attribute within an object   should be a subset of the object’s lifespan.