libenchao commented on code in PR #2848:
URL: https://github.com/apache/calcite/pull/2848#discussion_r915379084
##########
core/src/main/java/org/apache/calcite/rel/rules/SemiJoinRule.java:
##########
@@ -42,14 +43,18 @@
/**
* Planner rule that creates a {@code SemiJoin} from a
* {@link org.apache.calcite.rel.core.Join} on top of a
- * {@link org.apache.calcite.rel.logical.LogicalAggregate}.
+ * {@link org.apache.calcite.rel.logical.LogicalAggregate} or
+ * on a {@link org.apache.calcite.rel.RelNode} which is
+ * unique for join's right keys.
*/
public abstract class SemiJoinRule
extends RelRule<SemiJoinRule.Config>
implements TransformationRule {
private static boolean isJoinTypeSupported(Join join) {
final JoinRelType type = join.getJoinType();
- return type == JoinRelType.INNER || type == JoinRelType.LEFT;
+ return type == JoinRelType.INNER
+ || type == JoinRelType.LEFT
+ || type == JoinRelType.SEMI;
}
Review Comment:
It's good to know there is already a discussion about this before in
CALCITE-4623.
Consider this case, we have a plan below:
```xml
LogicalJoin(condition=[=($7, $9)], joinType=[inner])
LogicalTableScan(table=[[CATALOG, SALES, EMP]])
LogicalAggregate(group=[{0}], agg#0=[MIN($1)])
LogicalProject(DEPTNO=[$0], $f0=[true])
LogicalTableScan(table=[[CATALOG, SALES, DEPT]])
```
After `JoinOnUniqueToSemiJoinRule`, it would be transformed to:
```xml
LogicalJoin(condition=[=($7, $9)], joinType=[semi])
LogicalTableScan(table=[[CATALOG, SALES, EMP]])
LogicalAggregate(group=[{0}], agg#0=[MIN($1)])
LogicalProject(DEPTNO=[$0], $f0=[true])
LogicalTableScan(table=[[CATALOG, SALES, DEPT]])
```
And we can still use `JoinToSemiJoinRule` to remove the `Aggregate`:
```xml
LogicalJoin(condition=[=($7, $9)], joinType=[semi])
LogicalTableScan(table=[[CATALOG, SALES, EMP]])
LogicalProject(DEPTNO=[$0], $f0=[true])
LogicalTableScan(table=[[CATALOG, SALES, DEPT]])
```
In many cases, we would not expect the rules firing orders, especially in
`HepPlanner`, hence the above transforming order is valid and should be valid
IMHO.
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