hsyuan commented on a change in pull request #2006:
URL: https://github.com/apache/calcite/pull/2006#discussion_r439790064



##########
File path: 
core/src/main/java/org/apache/calcite/adapter/enumerable/EnumerableMergeJoin.java
##########
@@ -134,31 +137,83 @@ public static boolean isMergeJoinSupported(JoinRelType 
joinType) {
       final RelTraitSet required) {
     // Required collation keys can be subset or superset of merge join keys.
     RelCollation collation = required.getCollation();
+    int leftInputFieldCount = left.getRowType().getFieldCount();
+
     List<Integer> reqKeys = RelCollations.ordinals(collation);
-    ImmutableBitSet reqKeySet = ImmutableBitSet.of(reqKeys);
+    List<Integer> leftKeys = joinInfo.leftKeys.toIntegerList();
+    List<Integer> rightKeys =
+        joinInfo.rightKeys.incr(leftInputFieldCount).toIntegerList();
+    List<Integer> rightKeysNotShifted = joinInfo.rightKeys.toIntegerList();
 
+    ImmutableBitSet reqKeySet = ImmutableBitSet.of(reqKeys);
     ImmutableBitSet leftKeySet = ImmutableBitSet.of(joinInfo.leftKeys);
     ImmutableBitSet rightKeySet = ImmutableBitSet.of(joinInfo.rightKeys)
-        .shift(left.getRowType().getFieldCount());
+        .shift(leftInputFieldCount);
 
-    // Only consider exact key match for now
     if (reqKeySet.equals(leftKeySet)) {
+      // if sort keys equal to left join keys, we can pass through all 
collations directly.
       Mappings.TargetMapping mapping = buildMapping(true);
       RelCollation rightCollation = collation.apply(mapping);
       return Pair.of(
           required, ImmutableList.of(required,
           required.replace(rightCollation)));
+    } else if (containsOrderless(collation, leftKeys)) {

Review comment:
       How can it be true if sort keys are subset of left join keys?

##########
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);

Review comment:
       Why not just use bitset.except to compute the diff keys?




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