FatLittle commented on a change in pull request #1991:
URL: https://github.com/apache/calcite/pull/1991#discussion_r450588879



##########
File path: 
core/src/main/java/org/apache/calcite/plan/volcano/IterativeRuleDriver.java
##########
@@ -0,0 +1,87 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.calcite.plan.volcano;
+
+import org.apache.calcite.rel.RelNode;
+import org.apache.calcite.util.trace.CalciteTrace;
+
+import org.slf4j.Logger;
+
+/***
+ * <p>The algorithm executes repeatedly in a series of phases. In each phase
+ * the exact rules that may be fired varies. The mapping of phases to rule
+ * sets is maintained in the {@link #ruleQueue}.
+ *
+ * <p>In each phase, the planner then iterates over the rule matches presented
+ * by the rule queue until the rule queue becomes empty.
+ */
+class IterativeRuleDriver implements RuleDriver {
+
+  private static final Logger LOGGER = CalciteTrace.getPlannerTracer();
+
+  private final VolcanoPlanner planner;
+  private final VolcanoRuleQueue ruleQueue;
+
+  IterativeRuleDriver(VolcanoPlanner planner) {
+    this.planner = planner;
+    ruleQueue = new VolcanoRuleQueue(planner);
+  }
+
+  @Override public VolcanoRuleQueue getRuleQueue() {
+    return ruleQueue;
+  }
+
+  @Override public void drive() {
+    PLANNING:
+    for (VolcanoPlannerPhase phase : VolcanoPlannerPhase.values()) {
+      while (true) {
+        LOGGER.debug("PLANNER = {}; PHASE = {}; COST = {}",
+            this, phase.toString(), planner.root.bestCost);
+
+        VolcanoRuleMatch match = ruleQueue.popMatch(phase);
+        if (match == null) {
+          break;
+        }
+
+        assert match.getRule().matches(match);
+        try {
+          match.onMatch();
+        } catch (VolcanoTimeoutException e) {
+          planner.canonize();
+          ruleQueue.phaseCompleted(phase);
+          break PLANNING;

Review comment:
       this code is completely copied from previous findBestExpr method




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