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,
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+ * 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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