[GitHub] [calcite] korlov42 commented on a change in pull request #2623: CALCITE-4913 Correlated variables in a select list are not deduplicated

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korlov42 commented on a change in pull request #2623:
URL: https://github.com/apache/calcite/pull/2623#discussion_r761884509



##########
File path: core/src/main/java/org/apache/calcite/rel/logical/LogicalProject.java
##########
@@ -35,52 +37,63 @@
 import org.apache.calcite.util.Util;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
 
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.List;
+import java.util.Objects;
+import java.util.Set;
 
 /**
  * Sub-class of {@link org.apache.calcite.rel.core.Project} not
  * targeted at any particular engine or calling convention.
  */
 public final class LogicalProject extends Project {
+  private final ImmutableSet<CorrelationId> variablesSet;

Review comment:
       It does make sense for project with correlated scalar queries. For example, the query like `SELECT t0.v, (SELECT t0.v + t1.v FROM test_tbl t1) AS j FROM test_tbl t0` has a logical plan:
   ```
   LogicalProject(V=[$2], J=[$SCALAR_QUERY({
   LogicalProject(T0.V + T1.V=[+($cor0.V, $2)])
     IgniteLogicalTableScan(table=[[PUBLIC, TEST_TBL]])
   })])
     IgniteLogicalTableScan(table=[[PUBLIC, TEST_TBL]])
   ```
   In order to evaluate this plan you need either implement filter that set correlation variable, or convert it to LogicalCorrelate (what is actually done by `SubQueryRemoveRule` ).
   
   With that said, it seems to me LogicalFilter has to expose the variables they set, but not every physical implementation would do so.




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