Github user rxin commented on a diff in the pull request: https://github.com/apache/spark/pull/16431#discussion_r95103316 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/estimation/AggregateEstimation.scala --- @@ -0,0 +1,59 @@ +/* + * 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.estimation + +import org.apache.spark.sql.catalyst.expressions.Attribute +import org.apache.spark.sql.catalyst.plans.logical.{Aggregate, Statistics} + + +object AggregateEstimation { + import EstimationUtils._ + + def estimate(agg: Aggregate): Option[Statistics] = { + val childStats = agg.child.statistics + // Check if we have column stats for all group-by columns. + val colStatsExist = agg.groupingExpressions.forall { e => + e.isInstanceOf[Attribute] && childStats.attributeStats.contains(e.asInstanceOf[Attribute]) + } + if (rowCountsExist(agg.child) && colStatsExist) { + // Initial value for agg without group expressions + var outputRows: BigInt = 1 + agg.groupingExpressions.map(_.asInstanceOf[Attribute]).foreach { attr => + val colStat = childStats.attributeStats(attr) + // Multiply distinct counts of group by columns. This is an upper bound, which assumes + // the data contains all combinations of distinct values of group by columns. + outputRows *= colStat.distinctCount + } + + // The number of output rows must not be larger than child's number of rows. + // Note that this also covers the case of uniqueness of column. If one of the group-by columns --- End diff -- OK I don't think you need this explanation here -- it simply makes it more confusing. You are just putting an upper bound on cardinality, and that explains everything.
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