[GitHub] [spark] dbtsai commented on a change in pull request #23964: [SPARK-26975][SQL] Support nested-column pruning over limit/sample/repartition

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dbtsai commented on a change in pull request #23964: [SPARK-26975][SQL] Support nested-column pruning over limit/sample/repartition
URL: https://github.com/apache/spark/pull/23964#discussion_r267498207
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/NestedColumnAliasing.scala
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+/*
+ * 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.optimizer
+
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.plans.logical._
+import org.apache.spark.sql.internal.SQLConf
+import org.apache.spark.sql.types._
+
+/**
+ * This aims to handle a nested column aliasing pattern inside the `ColumnPruning` optimizer rule.
+ * If a project or its child references to nested fields, and not all the fields
+ * in a nested attribute are used, we can substitute them by alias attributes; then a project
+ * of the nested fields as aliases on the children of the child will be created.
+ */
+object NestedColumnAliasing {
+
+  def unapply(plan: LogicalPlan)
+    : Option[(Map[GetStructField, Alias], Map[ExprId, Seq[Alias]])] = plan match {
+    case Project(projectList, child)
+        if SQLConf.get.nestedSchemaPruningEnabled && canProjectPushThrough(child) =>
 
 Review comment:
   Agree with @viirya , this rule can be useful even for datasources that do't support nested pruning. We can have a followup to make it as a separate conf to enable it.

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