Class JoinPushThroughJoinRule

All Implemented Interfaces:

@Enclosing public class JoinPushThroughJoinRule extends RelRule<JoinPushThroughJoinRule.Config> implements TransformationRule
Rule that pushes the right input of a join into through the left input of the join, provided that the left input is also a join.

Thus, (A join B) join C becomes (A join C) join B. The advantage of applying this rule is that it may be possible to apply conditions earlier. For instance,

(sales as s join product_class as pc on true)
 join product as p
 on s.product_id = p.product_id
 and p.product_class_id = pc.product_class_id


(sales as s join product as p on s.product_id = p.product_id)
 join product_class as pc
 on p.product_class_id = pc.product_class_id

Before the rule, one join has two conditions and the other has none (ON TRUE). After the rule, each join has one condition.