[GitHub] cloud-fan commented on a change in pull request #23390: [SPARK-26459][SQL] replace UpdateNullabilityInAttributeReferences with FixNullability

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cloud-fan commented on a change in pull request #23390: [SPARK-26459][SQL] replace UpdateNullabilityInAttributeReferences with FixNullability
URL: https://github.com/apache/spark/pull/23390#discussion_r244324123
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/UpdateAttributeNullability.scala
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 @@ -0,0 +1,62 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.analysis
+
+import org.apache.spark.sql.catalyst.expressions.{Attribute, AttributeMap}
+import org.apache.spark.sql.catalyst.plans.logical.{LeafNode, LogicalPlan}
+import org.apache.spark.sql.catalyst.rules.Rule
+
+/**
+ * Updates nullability of Attributes in a resolved LogicalPlan by using the nullability of
+ * corresponding Attributes of its children output Attributes. This step is needed because
+ * users can use a resolved AttributeReference in the Dataset API and outer joins
+ * can change the nullability of an AttribtueReference. Without this rule, a nullable column's
+ * nullable field can be actually set as non-nullable, which cause illegal optimization
+ * (e.g., NULL propagation) and wrong answers.
+ * See SPARK-13484 and SPARK-13801 for the concrete queries of this case.
+ *
+ * This rule should be executed again at the end of optimization phase, as optimizer may change
+ * some expressions and their nullabilities as well. See SPARK-21351 for more details.
+ */
+object UpdateAttributeNullability extends Rule[LogicalPlan] {
+
+  def apply(plan: LogicalPlan): LogicalPlan = plan resolveOperatorsUp {
+    // Skip unresolved nodes.
+    case p if !p.resolved => p
 
 Review comment:
   no we don't. But it has no harm, and help us to merge these 2 rules.

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