morrySnow opened a new pull request, #13886:
URL: https://github.com/apache/doris/pull/13886
# Proposed changes
This rule eliminate project that output set is same with its child. If the
project is the root of plan, the elimination condition is project's output is
exactly the same with its child.
The reason to add this rule is when we do join reorder in optimization, the
root of plan after transformed maybe a Project and its output set is same with
the root of plan before transformed. If we had a Project on the top of the root
and its output set is same with the root of plan too. We will have two exactly
same projects in memo. One of them is the parent of the other. After
MergeProject, we will get a new Project exactly same like the child and need to
add to parent's group. Then we trigger Merge Group. Since merge will produce a
cycle, the merge will be denied and we will get a final plan with two
consecutive projects.
for example:
**BEFORE OPTIMIZATION**
```
LogicalProject1( projects=[c_custkey#0, c_name#1]) [GroupId#1]
+--LogicalJoin(type=LEFT_SEMI_JOIN) [GroupId#2]
|--LogicalProject(...)
| +--LogicalJoin(type=INNER_JOIN)
| ...
+--LogicalOlapScan(...)
```
**AFTER APPLY RULE: LOGICAL_SEMI_JOIN_LOGICAL_JOIN_TRANSPOSE_PROJECT**
```
LogicalProject1( projects=[c_custkey#0, c_name#1]) [GroupId#1]
+--LogicalProject2( projects=[c_custkey#0, c_name#1]) [GroupId#2]
+--LogicalJoin(type=INNER_JOIN) [GroupId#10]
|--LogicalProject(...)
| +--LogicalJoin(type=LEFT_SEMI_JOIN)
| ...
+--LogicalOlapScan(...)
```
**AFTER APPLY RULE: MERGE_PROJECTS**
```
LogicalProject3( projects=[c_custkey#0, c_name#1]) [should be in GroupId#1,
but in GroupId#2 in fact]
+--LogicalJoin(type=INNER_JOIN) [GroupId#10]
|--LogicalProject(...)
| +--LogicalJoin(type=LEFT_SEMI_JOIN)
| ...
+--LogicalOlapScan(...)
```
Since we have exaclty GroupExpression(LogicalProject3 and LogicalProject2)
in GroupId#1 and GroupId#2, we need to do MergeGroup(GroupId#1, GroupId#2). But
we have child of GroupId#1 in GroupId#2. So the merge is denied.
If the best GroupExpression in GroupId#2 is LogicalProject3, we will get two
consecutive projects in the final plan.
## Problem summary
Describe your changes.
## Checklist(Required)
1. Does it affect the original behavior:
- [ ] Yes
- [ ] No
- [ ] I don't know
2. Has unit tests been added:
- [ ] Yes
- [ ] No
- [ ] No Need
3. Has document been added or modified:
- [ ] Yes
- [ ] No
- [ ] No Need
4. Does it need to update dependencies:
- [ ] Yes
- [ ] No
5. Are there any changes that cannot be rolled back:
- [ ] Yes (If Yes, please explain WHY)
- [ ] No
## Further comments
If this is a relatively large or complex change, kick off the discussion at
[d...@doris.apache.org](mailto:d...@doris.apache.org) by explaining why you
chose the solution you did and what alternatives you considered, etc...
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