maropu 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_r265849055
########## File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/NestedColumnAliasing.scala ########## @@ -0,0 +1,150 @@ +/* + * 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.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(_, child) if canProjectPushThrough(child) => + getAliasSubMap(plan, child) + case _ => None + } + + /** + * Replace nested columns to prune unused nested columns later. + */ + def replaceToAliases( + plan: LogicalPlan, + nestedFieldToAlias: Map[GetStructField, Alias], + attrToAliases: Map[ExprId, Seq[Alias]]): LogicalPlan = plan match { + case Project(projectList, child) => + Project( + getNewProjectList(projectList, nestedFieldToAlias), + replaceChildrenWithAliases(child, attrToAliases)) + } + + /** + * Return a replaced project list. + */ + private def getNewProjectList( + projectList: Seq[NamedExpression], + nestedFieldToAlias: Map[GetStructField, Alias]): Seq[NamedExpression] = { + projectList.map(_.transform { + case f: GetStructField if nestedFieldToAlias.contains(f) => + nestedFieldToAlias(f).toAttribute + }.asInstanceOf[NamedExpression]) + } + + /** + * Return a plan with new childen replaced with aliases. + */ + private def replaceChildrenWithAliases( + plan: LogicalPlan, + attrToAliases: Map[ExprId, Seq[Alias]]): LogicalPlan = { + plan.withNewChildren(plan.children.map { plan => + Project(plan.output.flatMap(a => attrToAliases.getOrElse(a.exprId, Seq(a))), plan) + }) + } + + /** + * Returns true for those operators that project can be pushed through. + */ + private def canProjectPushThrough(plan: LogicalPlan) = plan match { + case _: GlobalLimit => true + case _: LocalLimit => true + case _: Repartition => true + case _: Sample => true + case _ => false + } + + /** + * Return root references that are individually accessed as a whole, and `GetStructField`s. + */ + private def collectRootReferenceAndGetStructField(plan: LogicalPlan): Seq[Expression] = { + def helper(e: Expression): Seq[Expression] = e match { + case _: AttributeReference | _: GetStructField => Seq(e) + case es if es.children.nonEmpty => es.children.flatMap(helper) + case _ => Seq.empty + } + plan.expressions.flatMap(helper) + } + + /** + * Return two maps in order to replace nested fields to aliases. + * + * 1. GetStructField -> Alias: A new alias is created for each nested field. + * 2. ExprId -> Seq[Alias]: A reference attribute has multiple aliases pointing it. + */ + private def getAliasSubMap(plans: LogicalPlan*) + : Option[(Map[GetStructField, Alias], Map[ExprId, Seq[Alias]])] = { + val (nestedFieldReferences, otherRootReferences) = plans + .map(collectRootReferenceAndGetStructField).reduce(_ ++ _).partition { + case _: GetStructField => true + case _ => false + } + + val aliasSub = nestedFieldReferences.asInstanceOf[Seq[GetStructField]] + .filter(!_.references.subsetOf(AttributeSet(otherRootReferences))) + .groupBy(_.references.head) + .flatMap { case (attr: Attribute, nestedFields: Seq[GetStructField]) => + // Each expression can contain multiple nested fields. + // Note that we keep the original names to deliver to parquet in a case-sensitive way. + val nestedFieldToAlias = nestedFields.distinct.map { f => + val exprId = NamedExpression.newExprId + (f, Alias(f, s"_gen_alias_${exprId.id}")(exprId, Seq.empty, None)) + } + + if (nestedFieldToAlias.nonEmpty && + nestedFieldToAlias.length < totalFieldNum(attr.dataType)) { Review comment: Could you leave comments here about why we need the check: `nestedFieldToAlias.length < totalFieldNum(attr.dataType)`? ---------------------------------------------------------------- This is an automated message from the Apache Git Service. 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