9.2. Dependent Join Pushdown

Dependent joins are a technique used in federation to reduce the cost of cross source joins. Join values from one side of a join are made available to the other side which reduces the number of tuples needed to preform the join. Translators may indicate support for dependent join pushdown via the supportsDependentJoin capability. The handling of pushdown dependent join queries can be quite complicated. The ordering (if present) and all of the non-dependent criteria constructs on the pushdown command must be honored, but if needed the dependent criteria, which will be a Comparison with a Parameter, may be ignored in part or in total. Pushdown dependent join queries will be instances of Select with the relevant dependent sets available via Select.getDependentSets(). The dependent set is associated to Parameters by id via the Parameter.getDepenentValueId() identifier. The dependent set tuple iterators provide rows that are referenced by the column positions (available via Parameter.getValueIndex() ) on the dependent join Comparison criteria right expression. Care should be taken with the tuple values as they may guaranteed to be unique or ordered.