imback82 commented on a change in pull request #28123:
URL: https://github.com/apache/spark/pull/28123#discussion_r417052075



##########
File path: 
sql/core/src/main/scala/org/apache/spark/sql/execution/bucketing/CoalesceBucketsInJoin.scala
##########
@@ -0,0 +1,116 @@
+/*
+ * 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.bucketing
+
+import org.apache.spark.sql.catalyst.catalog.BucketSpec
+import org.apache.spark.sql.catalyst.plans.Inner
+import org.apache.spark.sql.catalyst.plans.logical.{Filter, Join, LogicalPlan, 
Project}
+import org.apache.spark.sql.catalyst.rules.Rule
+import org.apache.spark.sql.execution.datasources.{HadoopFsRelation, 
LogicalRelation}
+import org.apache.spark.sql.internal.SQLConf
+
+/**
+ * This rule injects a hint if one side of two bucketed tables can be coalesced
+ * when the two bucketed tables are inner-joined and they differ in the number 
of buckets.
+ */
+object CoalesceBucketsInJoin extends Rule[LogicalPlan]  {
+  private def isPlanEligible(plan: LogicalPlan): Boolean = {
+    def forall(plan: LogicalPlan)(p: LogicalPlan => Boolean): Boolean = {
+      p(plan) && plan.children.forall(forall(_)(p))
+    }
+
+    forall(plan) {
+      case _: Filter | _: Project | _: LogicalRelation => true
+      case _ => false
+    }
+  }
+
+  private def getBucketSpec(plan: LogicalPlan): Option[BucketSpec] = {
+    if (isPlanEligible(plan)) {
+      plan.collectFirst {
+        case _ @ LogicalRelation(r: HadoopFsRelation, _, _, _) if 
r.bucketSpec.nonEmpty =>
+          r.bucketSpec.get
+      }
+    } else {
+      None
+    }
+  }
+
+  private def mayCoalesce(
+      numBuckets1: Int,
+      numBuckets2: Int,
+      maxNumBucketsDiff: Int): Option[Int] = {

Review comment:
       I changed it to pass sqlConf (I guess that's what you were leaning 
toward?)




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