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. Only ordering on collations defined on key need to preserve. For
example, required collation [foo.a, foo.b, foo.c ,foo.d] defined on
`foo.a=bar.a and foo.b=bar.b`, 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 (but [a, b]'s
ordering must remain).
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 could be 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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