Github user ericl commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15539#discussion_r83986875
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileStatusCache.scala
 ---
    @@ -0,0 +1,78 @@
    +/*
    + * 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.datasources
    +
    +import java.util.concurrent.ConcurrentHashMap
    +
    +import org.apache.hadoop.fs.{FileStatus, Path}
    +
    +import org.apache.spark.metrics.source.HiveCatalogMetrics
    +import org.apache.spark.util.SerializableConfiguration
    +
    +/**
    + * A cache of the leaf files of partition directories. We cache these 
files in order to speed
    + * up iterated queries over the same set of partitions. Otherwise, each 
query would have to
    + * hit remote storage in order to gather file statistics for physical 
planning.
    + *
    + * Each resolved catalog table has its own FileStatusCache. When the 
backing relation for the
    + * table is refreshed via refreshTable() or refreshByPath(), this cache 
will be invalidated.
    + */
    +abstract class FileStatusCache {
    +  /**
    +   * @return the leaf files for the specified path from this cache, or 
None if not cached.
    +   */
    +  def getLeafFiles(path: Path): Option[Array[FileStatus]] = None
    +
    +  /**
    +   * Saves the given set of leaf files for a path in this cache.
    +   */
    +  def putLeafFiles(path: Path, leafFiles: Array[FileStatus]): Unit
    +
    +  /**
    +   * Invalidates all data held by this cache.
    +   */
    +  def invalidateAll(): Unit
    +}
    +
    +/**
    + * An implementation that caches all partition file statuses in memory 
forever.
    + */
    +class InMemoryCache extends FileStatusCache {
    +  private val cache = new ConcurrentHashMap[Path, Array[FileStatus]]()
    +
    +  override def getLeafFiles(path: Path): Option[Array[FileStatus]] = {
    +    Option(cache.get(path))
    +  }
    +
    +  override def putLeafFiles(path: Path, leafFiles: Array[FileStatus]): 
Unit = {
    +    cache.put(path, leafFiles.toArray)
    +  }
    +
    +  override def invalidateAll(): Unit = {
    +    cache.clear()
    +  }
    +}
    +
    +/**
    + * A non-caching implementation used when partition file status caching is 
disabled.
    + */
    +class NoopCache extends FileStatusCache {
    --- End diff --
    
    good idea


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