6f3db8b4b4
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.
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