5.2.2 Closure tables
A closure table is a SQL table which contains a record for every employee/supervisor relationship, regardless of depth. (In mathematical terms, this is called the 'reflexive transitive closure' of the employee/supervisor relationship. The distance column is not strictly required, but it makes it easier to populate the table.)
| employee_closure | ||
| supervisor_id | employee_id | distance |
| 1 | 1 | 0 |
| 1 | 2 | 1 |
| 1 | 3 | 2 |
| 1 | 4 | 1 |
| 1 | 5 | 3 |
| 1 | 6 | 2 |
| 2 | 2 | 0 |
| 2 | 3 | 1 |
| 2 | 5 | 2 |
| 2 | 6 | 1 |
| 3 | 3 | 0 |
| 3 | 5 | 1 |
| 4 | 4 | 0 |
| 5 | 5 | 0 |
| 6 | 6 | 0 |
In the catalog XML, the <Closure> element maps the level onto a <Table>:
< Dimension name="Employees" foreignKey="employee_id">< Hierarchy hasAll="true" allMemberName="All Employees" primaryKey="employee_id">< Table name="employee"/>< Level name="Employee Id" uniqueMembers="true" type="Numeric"
column="employee_id" nameColumn="full_name" parentColumn="supervisor_id" nullParentValue="0">< Closure parentColumn="supervisor_id" childColumn="employee_id">< Table name="employee_closure"/></ Closure>< Property name="Marital Status" column="marital_status"/>< Property name="Position Title" column="position_title"/>< Property name="Gender" column="gender"/>< Property name="Salary" column="salary"/>< Property name="Education Level" column="education_level"/>< Property name="Management Role" column="management_role"/></ Level></ Hierarchy></ Dimension>
This table allows totals to be evaluated in pure SQL. Even though this introduces an extra table into the query, database optimizers are very good at handling joins. I recommend that you declare both supervisor_id and employee_id NOT NULL, and index them as follows:
CREATE UNIQUE INDEX employee_closure_pk ON employee_closure (
supervisor_id,
employee_id);
CREATE INDEX employee_closure_emp ON employee_closure (
employee_id);
5.2.3 Populating closure tables
The table needs to be re-populated whenever the hierarchy changes, and it is the application's responsibility to do so — Mondrian does not do this!
If you are using Pentaho Data Integration (Kettle), there is a special step to populate closure tables as part of the ETL process. Further details in the Pentaho Data Integration wiki.
 Closure Generator step in Pentaho Data Integration |
If you are not using Pentaho Data Integration, you can populate the table yourself using SQL. Here is an example of a MySQL stored procedure that populates a closure table.
DELIMITER //
CREATE PROCEDURE populate_employee_closure()
BEGIN
DECLARE distance int;
TRUNCATE TABLE employee_closure;
SET distance = 0;
-- seed closure with self-pairs (distance 0)
INSERT INTO employee_closure (supervisor_id, employee_id, distance)
SELECT employee_id, employee_id, distance
FROM employee;
-- for each pair (root, leaf) in the closure,
-- add (root, leaf->child) from the base table
REPEAT
SET distance = distance + 1;
INSERT INTO employee_closure (supervisor_id, employee_id, distance)
SELECT employee_closure.supervisor_id, employee.employee_id, distance
FROM employee_closure, employee
WHERE employee_closure.employee_id = employee.supervisor_id
AND employee_closure.distance = distance - 1;
UNTIL (ROW_COUNT() == 0))
END REPEAT;
END //
DELIMITER ;