Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/13494#discussion_r68536010
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/MetadataOnlyOptimizer.scala
---
@@ -0,0 +1,190 @@
+/*
+ * 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.execution
+
+import org.apache.spark.sql.{AnalysisException, SparkSession}
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.catalog.{CatalogRelation,
SessionCatalog}
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.expressions.aggregate._
+import org.apache.spark.sql.catalyst.plans.logical._
+import org.apache.spark.sql.catalyst.rules.Rule
+import org.apache.spark.sql.execution.datasources.{HadoopFsRelation,
LogicalRelation}
+
+/**
+ * When scanning only partition columns, get results based on metadata
without scanning files.
+ * It is used for distinct, distinct aggregations or distinct-like
aggregations(example: Max/Min).
+ * Example: select Max(partition) from table.
+ */
+case class MetadataOnlyOptimizer(
+ sparkSession: SparkSession,
+ catalog: SessionCatalog) extends Rule[LogicalPlan] {
+
+ private def canSupportMetadataOnly(a: Aggregate): Boolean = {
--- End diff --
here is my thoughts about the optimizable cases:
First of all, only parition colums are required(which means we need to
traverse down the plan tree and find table relation here)
1. aggregate expression is partition columns, e.g. `SELECT col FROM tbl
GROUP BY col`
2. aggregate function on partition columns with DISTINCT, e.g. `SELECT
count(DISTINCT a) FROM tbl GROUP BY b`
3. aggregate function on partition columns which have same result with or
without DISTINCT keyword, e.g. `SELECT sum(a) FROM tbl GROUP BY b`
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