Github user cloud-fan commented on a diff in the pull request: https://github.com/apache/spark/pull/16430#discussion_r95098331 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/statsEstimation/ProjectEstimation.scala --- @@ -0,0 +1,44 @@ +/* + * 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.plans.logical.statsEstimation + +import org.apache.spark.sql.catalyst.expressions.{Alias, Attribute, AttributeMap} +import org.apache.spark.sql.catalyst.plans.logical.{Project, Statistics} + +object ProjectEstimation { + import EstimationUtils._ + + def estimate(project: Project): Option[Statistics] = { + if (rowCountsExist(project.child)) { + val childStats = project.child.statistics + val inputAttrStats = childStats.attributeStats + // Match alias with its child's column stat + val aliasStats = project.expressions.collect { + case alias @ Alias(attr: Attribute, _) if inputAttrStats.contains(attr) => --- End diff -- In the long run, we should define a `statistics` interface in `Expression`, so that we can propagate the column stats more naturally, for more cases(not only `Alias`, but also `Add`, `Mod`, etc.). But currently catalyst doesn't propagate attributes correctly, e.g. https://issues.apache.org/jira/browse/SPARK-17995 (`Union`, `Except`, etc. has the same problem), we may need to hack a lot of places to propagate column stats correctly. According to @wzhfy 's benchmark, it turns out we can speed up most of the cases if we take care of `Alias`, so I'm ok with the current approach.
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