[CALCITE-926] Rules fail to match because of missing link to parent equivalence set

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.5.0
• Assignee: Wei Xue
• Reporter: Wei Xue
• Created: 19/Oct/15 17:28
• Updated: 27/Feb/24 22:24
• Resolved: 23/Oct/15 19:40

Description

There is no "parent link" in RelSubset, it is only for RelSet. However, RelSubset provides a "getParentRels()" link.

  public Collection<RelNode> getParentRels() {
    final Set<RelNode> list = new LinkedHashSet<RelNode>();
  parentLoop:
    for (RelNode parent : set.getParentRels()) {
      for (RelSubset rel : inputSubsets(parent)) {
        if (rel.set == set && rel.getTraitSet().equals(traitSet)) {
          list.add(parent);
          continue parentLoop;
        }
      }
    }
    return list;
  }

Now that we have a RelNode P1 and its input subset is S1, and we have a rule that matches P1 and does the following:

onMatch(RelOptRuleCall call) {
  RelNode P1 = call.rel(0);
  RelNode S1 = P1.getInput(0);
  RelTraitSet t1 = S1.getTraitSet();
  RelTraitSet t2 = t1.replace(tx);
  RelNode S2 = convert(S1, t2);
  RelNode P2 = P1.copy(P1.getCluster, P1.getTraitSet(), S2);
  call.transformTo(P2);
}

Note that t1 is NOT equal to t2. So right now we have two subsets, S1 and S2, in the same set; and their parents P1 and P2 in another set.

P1            P2
|             |
S1(t1)        S2(t2)

getParentRels(S1) would return P1, and getParentRels(S2) would return P2. This works fine except when t1 satisfies t2 (for instance, if tx is the empty collation trait) and as a result S2 subsumes S1. Thus the expression S2 = convert(S1, t2) would eventually turn into S1 after conversion, so P2 is also a parent of S1, and getParentRels(S1) should return P1, P2 instead.
Suppose we have a second rule that should match P2 - R subsequently, R as some Rel in S1, it would NOT be fired since P2 is not returned in the parent list of S1. It would not match R in S2 either since it is in the form of P2 - AbstractConverter - R.