amaliujia commented on a change in pull request #2006:
URL: https://github.com/apache/calcite/pull/2006#discussion_r439784146



##########
File path: 
core/src/main/java/org/apache/calcite/adapter/enumerable/EnumerableMergeJoin.java
##########
@@ -222,6 +277,30 @@ public static boolean isMergeJoinSupported(JoinRelType 
joinType) {
     return mapping;
   }
 
+  private RelCollation extendCollation(RelCollation collation, List<Integer> 
keys) {
+    List<RelFieldCollation> fieldsForNewCollation = new 
ArrayList<>(keys.size());
+    fieldsForNewCollation.addAll(collation.getFieldCollations());
+    Set<Integer> keySet = new HashSet<>(keys);
+    for (RelFieldCollation rf : collation.getFieldCollations()) {
+      keySet.remove(rf.getFieldIndex());
+    }
+    for (Integer i : keySet) {
+      fieldsForNewCollation.add(new RelFieldCollation(i));
+    }
+    return RelCollations.of(fieldsForNewCollation);
+  }
+
+  private RelCollation removeCollationFieldsNotOnJoinKey(

Review comment:
       Let me add some comments with an example (a little bit complicated but 
let me try).




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