Class JoinToCorrelateRule


  • public class JoinToCorrelateRule
    extends RelOptRule
    Rule that converts a Join into a LogicalCorrelate, which can then be implemented using nested loops.

    For example,

    select * from emp join dept on emp.deptno = dept.deptno

    becomes a Correlator which restarts LogicalTableScan("DEPT") for each row read from LogicalTableScan("EMP").

    This rule is not applicable if for certain types of outer join. For example,

    select * from emp right join dept on emp.deptno = dept.deptno

    would require emitting a NULL emp row if a certain department contained no employees, and Correlator cannot do that.

    • Constructor Detail

      • JoinToCorrelateRule

        private JoinToCorrelateRule​(java.lang.Class<? extends Join> clazz,
                                    RelBuilderFactory relBuilderFactory,
                                    java.lang.String description)
        Creates a JoinToCorrelateRule for a certain sub-class of Join to be transformed into a LogicalCorrelate.
        Parameters:
        clazz - Class of relational expression to match (must not be null)
        relBuilderFactory - Builder for relational expressions
        description - Description, or null to guess description
    • Method Detail

      • matches

        public boolean matches​(RelOptRuleCall call)
        Description copied from class: RelOptRule
        Returns whether this rule could possibly match the given operands.

        This method is an opportunity to apply side-conditions to a rule. The RelOptPlanner calls this method after matching all operands of the rule, and before calling RelOptRule.onMatch(RelOptRuleCall).

        In implementations of RelOptPlanner which may queue up a matched RelOptRuleCall for a long time before calling RelOptRule.onMatch(RelOptRuleCall), this method is beneficial because it allows the planner to discard rules earlier in the process.

        The default implementation of this method returns true. It is acceptable for any implementation of this method to give a false positives, that is, to say that the rule matches the operands but have RelOptRule.onMatch(RelOptRuleCall) subsequently not generate any successors.

        The following script is useful to identify rules which commonly produce no successors. You should override this method for these rules:

        awk '
         /Apply rule/ {rule=$4; ruleCount[rule]++;}
         /generated 0 successors/ {ruleMiss[rule]++;}
         END {
           printf "%-30s %s %s\n", "Rule", "Fire", "Miss";
           for (i in ruleCount) {
             printf "%-30s %5d %5d\n", i, ruleCount[i], ruleMiss[i];
           }
         } ' FarragoTrace.log
        Overrides:
        matches in class RelOptRule
        Parameters:
        call - Rule call which has been determined to match all operands of this rule
        Returns:
        whether this RelOptRule matches a given RelOptRuleCall