Exclusion Product Join Process
- For each left table row, read all right table rows from the beginning until one is found that can be joined with it.
- Produce the join result.
If no matching right table rows are found, return the left row.
Exclusion Product Join Example
The following request returns the names of those employees who do not work in Chicago:
SELECT name FROM employee WHERE dept_no NOT IN (SELECT dept_no FROM department WHERE loc = ’CHI’);
Because the subquery returns only one row, the Optimizer selects an exclusion product join for the join plan using with the following process:
- All AMPs are searched for department rows where loc = ‘CHI’.
- If only one AMP is selected, and if loc is an index, then an all-AMPs retrieve is not performed.
- The spool containing the single department row is duplicated on every AMP that contains the spooled employee rows.
- The single row found to satisfy this condition, that for department 600, is duplicated right away, without being spooled in the local AMP.
- The rows in the two spools undergo an exclusion product join on each AMP.
- Name information for any employee row whose dept_no is not 600 is placed in a result spool on each AMP.
- When the last AMP has completed its portion of the join, the contents of all results spools are sent to the requesting application via a BYNET merge.
The following graphic illustrates this process: