[GitHub] [calcite] jcamachor commented on a change in pull request #1185: [CALCITE-3012] areColumnsUnique for FULL OUTER JOIN could return wrong answer when ignoreNulls is false

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jcamachor commented on a change in pull request #1185: [CALCITE-3012] areColumnsUnique for FULL OUTER JOIN could return wrong answer when ignoreNulls is false
URL: https://github.com/apache/calcite/pull/1185#discussion_r280481086
 
 

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 File path: core/src/main/java/org/apache/calcite/rel/metadata/RelMdColumnUniqueness.java
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 @@ -254,6 +255,13 @@ public Boolean areColumnsUnique(Join rel, RelMetadataQuery mq,
     final ImmutableBitSet leftColumns = leftAndRightColumns.left;
     final ImmutableBitSet rightColumns = leftAndRightColumns.right;
 
+    // for FULL OUTER JOIN if columns contain column from both inputs it is not
+    // guranteed that the result will be unique
+    if (!ignoreNulls && rel.getJoinType() == JoinRelType.FULL
+        && leftColumns.cardinality() > 0 && rightColumns.cardinality() > 0) {
+      return false;
+    }
 
 Review comment:
   @vineetgarg02 , in the javadoc it is mentioned:
   ```
      * @return whether the columns are unique, or
      * null if not enough information is available to make that determination
   ```
   In this specific case, you are not sure whether columns are unique or not, hence I believe you should return null.

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