amaliujia commented on a change in pull request #2006:
URL: https://github.com/apache/calcite/pull/2006#discussion_r439784121
##########
File path:
core/src/main/java/org/apache/calcite/adapter/enumerable/EnumerableMergeJoin.java
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@@ -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:
No for subset cases. For example, required collation [foo.a, foo.b]
defined on `foo.a=bar.a and foo.b=bar.b and foo.c=bar.d and foo.d=bar.d`,
either pass down [foo.a, foo.b, foo.c, foo.d] or pass down [foo.a, foo.b,
foo.d, foo.c] will give correct answer (same to push the same collations to
right join input).
It is because, for MergeJoin implementation, the pointers moves when left
tuple not equals to right tuple, and the only requirement is next tuple should
be bigger than previous ones.
Still use example above:
left join input and right join input both are sorted by [a, b, d, c], and
right join input is sorted by [a, b]. So
1) `left_tuple.compare(right_tuple) < 0`, move left pointer, of course next
tuple will be both bigger than a) the previous left tuple b) the current right
tuple (due to they are all sorted by [a, b]).
2) same for ``left_tuple.compare(right_tuple) > 0` case.
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