[GitHub] [doris] morrySnow commented on a diff in pull request #15833: [fix](Nereids): add unusedFlag to avoid job run used GroupExprssion

[GitBox <gi...@apache.org>]

morrySnow commented on code in PR #15833:
URL: https://github.com/apache/doris/pull/15833#discussion_r1067844668


##########
fe/fe-core/src/main/java/org/apache/doris/nereids/memo/Memo.java:
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@@ -465,6 +468,8 @@ public Group mergeGroup(Group source, Group destination) {
             source.moveOwnership(destination);
             groups.remove(source.getGroupId());
         }
+
+        needMergeGroups.forEach(this::mergeGroup);

Review Comment:
   consider a scenario:
   if we have two entry in the map: B -> A and C -> B.
   when we do merge group. we first merge B to A, and then B is removed. after that, we want to merge C to B, but B is not exist anymore.
   So, maybe we need to use **disjoint set** do find a common parent, and then merge all other group to it in one **disjoint set**.



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