[CALCITE-3508] Strengthen outer Join to inner if it is under a Filter that discards null values

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Labels: pull-request-available
• Assignee: Unassigned
• Reporter: Scott Reynolds
• Created: 15/Nov/19 21:03
• Updated: 18/Nov/19 23:56

Description

Today, FilterJoinRule given an Outer Join the rule strengthens it to Inner Join when the nullable side contains a filter IS_NOT_NULL. Below is the code.

for (RexNode filter : aboveFilters) {
      if (joinType.generatesNullsOnLeft()
          && Strong.isNotTrue(filter, leftBitmap)) {
        joinType = joinType.cancelNullsOnLeft();
      }
      if (joinType.generatesNullsOnRight()
          && Strong.isNotTrue(filter, rightBitmap)) {
        joinType = joinType.cancelNullsOnRight();
      }
      if (!joinType.isOuterJoin()) {
        break;
      }
    }

This code looks at the filter to determine if it is always true, then it can alter the join type by removing the null on that side.

We can see this in the following test RelOptRules#testStrengthenJoinType, which executes the following SQL that transforms from a LEFT OUTER JOIN to an INNER JOIN

select *
from dept left join emp on dept.deptno = emp.deptno
where emp.deptno is not null and emp.sal > 100

This ticket is about broadening the application of this rule to a sql like the following:

select *
from dept left join emp on dept.deptno = emp.deptno
where emp.sal > 100

 This originally came up on the mailing list: https://mail-archives.apache.org/mod_mbox/calcite-dev/201909.mbox/%3CCAGzrZ38hh%2B8B8TG8gVHUfVKunJZ4L%2BK7rmrinQxpwOHcdJbzGQ%40mail.gmail.com%3E

and in that thread it was pointed out that there are filters that prevent this from being applied:

SELECT b.title
FROM Book b
LEFT JOIN Author a ON b.author = a.id
WHERE a.name <> 'Victor'

This means we need to ensure we that the OUTER JOIN doesn't contain – for lack of a different term – negation filters. If there is a negation – like NOT_EQUAL – the JOIN cannot be strengthened.

Issue Links

• links to: GitHub Pull Request #1589