englefly commented on code in PR #11717:
URL: https://github.com/apache/doris/pull/11717#discussion_r944188694
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fe/fe-core/src/main/java/org/apache/doris/nereids/memo/Memo.java:
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@@ -169,7 +169,34 @@ private Pair<Boolean, GroupExpression>
rewriteGroupExpression(
GroupExpression groupExpression, Group target, LogicalProperties
logicalProperties) {
boolean newGroupExpressionGenerated = true;
GroupExpression existedGroupExpression =
groupExpressions.get(groupExpression);
- if (existedGroupExpression != null) {
+ /*
+ * here we need to handle one situation that original target is not
the same with
+ * existedGroupExpression.getOwnerGroup(). In this case, if we change
target to
+ * existedGroupExpression.getOwnerGroup(), we could not rewrite plan
as we expected and the plan
+ * will not be changed anymore.
+ * Think below example:
+ * We have a plan like this:
+ * Original (Group 2 is root):
+ * Group2: Project(outside)
+ * Group1: |---Project(inside)
+ * Group0: |---UnboundRelation
+ *
+ * and we want to rewrite group 2 by Project(inside, GroupPlan(group
0))
+ *
+ * After rewriting we should get (Group 2 is root):
+ * Group2: Project(inside)
+ * Group0: |---UnboundRelation
+ *
+ * Group1: Project(inside)
+ *
+ * After rewriting, Group 1's GroupExpression is not in
GroupExpressionsMap anymore and Group 1 is unreachable.
+ * Merge Group 1 into Group 2 is better, but in consideration of there
is others way to let a Group take into
+ * unreachable. There's no need to complicate to add a merge step.
Instead, we need to have a clear step to
+ * remove unreachable groups and GroupExpressions after rewrite.
+ * TODO: add a clear groups function to memo.
+ */
+ if (existedGroupExpression != null
+ && (target == null ||
target.equals(existedGroupExpression.getOwnerGroup()))) {
target = existedGroupExpression.getOwnerGroup();
Review Comment:
if target is unreachable, it should be removed, `groups.remove(target)`,
right?
But if this target is a child of other GroupExpression, it is still
reachable.
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