Github user HyukjinKwon commented on a diff in the pull request:
https://github.com/apache/spark/pull/21320#discussion_r203931060
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/planning/ProjectionOverSchema.scala
---
@@ -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.planning
+
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.types._
+
+/**
+ * A Scala extractor that projects an expression over a given schema. Data
types,
+ * field indexes and field counts of complex type extractors and attributes
+ * are adjusted to fit the schema. All other expressions are left as-is.
This
+ * class is motivated by columnar nested schema pruning.
+ */
+case class ProjectionOverSchema(schema: StructType) {
+ private val fieldNames = schema.fieldNames.toSet
+
+ def unapply(expr: Expression): Option[Expression] = getProjection(expr)
+
+ private def getProjection(expr: Expression): Option[Expression] =
+ expr match {
+ case a @ AttributeReference(name, _, _, _) if
(fieldNames.contains(name)) =>
+ Some(a.copy(dataType = schema(name).dataType)(a.exprId,
a.qualifier))
+ case GetArrayItem(child, arrayItemOrdinal) =>
+ getProjection(child).map {
+ case projection =>
+ GetArrayItem(projection, arrayItemOrdinal)
--- End diff --
nit:
```
getProjection(child).map { projection => GetArrayItem(projection,
arrayItemOrdinal) }
```
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