alexeykudinkin commented on a change in pull request #4877:
URL: https://github.com/apache/hudi/pull/4877#discussion_r825222033



##########
File path: 
hudi-spark-datasource/hudi-spark-common/src/main/scala/org/apache/hudi/BaseFileOnlyRelation.scala
##########
@@ -0,0 +1,94 @@
+/*
+ * 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.hudi
+
+import org.apache.hadoop.conf.Configuration
+import org.apache.hadoop.fs.Path
+import org.apache.hudi.HoodieBaseRelation.createBaseFileReader
+import org.apache.hudi.common.table.HoodieTableMetaClient
+import org.apache.spark.sql.SQLContext
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions.Expression
+import org.apache.spark.sql.execution.datasources._
+import org.apache.spark.sql.sources.{BaseRelation, Filter}
+import org.apache.spark.sql.types.StructType
+
+/**
+ * [[BaseRelation]] implementation only reading Base files of Hudi tables, 
essentially supporting following querying
+ * modes:
+ * <ul>
+ * <li>For COW tables: Snapshot</li>
+ * <li>For MOR tables: Read-optimized</li>
+ * </ul>
+ *
+ * NOTE: The reason this Relation is used in liue of Spark's default 
[[HadoopFsRelation]] is primarily due to the
+ * fact that it injects real partition's path as the value of the partition 
field, which Hudi ultimately persists
+ * as part of the record payload. In some cases, however, partition path might 
not necessarily be equal to the
+ * verbatim value of the partition path field (when custom [[KeyGenerator]] is 
used) therefore leading to incorrect
+ * partition field values being written
+ */
+class BaseFileOnlyRelation(sqlContext: SQLContext,

Review comment:
       Interesting, that's a good point. Let me try to do exactly in that order 
next time. 
   
   Weirdly enough, not sure why order should matter in that case.

##########
File path: 
hudi-spark-datasource/hudi-spark-common/src/main/scala/org/apache/hudi/HoodieDataSourceHelper.scala
##########
@@ -65,20 +65,6 @@ object HoodieDataSourceHelper extends PredicateHelper {
     }
   }
 
-  /**
-   * Extract the required schema from [[InternalRow]]
-   */
-  def extractRequiredSchema(

Review comment:
       Correct

##########
File path: 
hudi-spark-datasource/hudi-spark-common/src/main/scala/org/apache/spark/HoodieHadoopFSUtils.scala
##########
@@ -0,0 +1,370 @@
+/*
+ * 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
+
+import org.apache.hadoop.conf.Configuration
+import org.apache.hadoop.fs.viewfs.ViewFileSystem
+import org.apache.hadoop.fs._
+import org.apache.hadoop.hdfs.DistributedFileSystem
+import org.apache.spark.internal.Logging
+import org.apache.spark.metrics.source.HiveCatalogMetrics
+import org.apache.spark.util.SerializableConfiguration
+
+import java.io.FileNotFoundException
+import scala.collection.mutable
+
+/**
+ * NOTE: This method class is replica of HadoopFSUtils from Spark 3.2.1, with 
the following adjustments
+ *
+ *    - Filtering out of the listed files is adjusted to include files 
starting w/ "." (to include Hoodie Delta Log
+ *    files)
+ */
+object HoodieHadoopFSUtils extends Logging {
+  /**
+   * Lists a collection of paths recursively. Picks the listing strategy 
adaptively depending
+   * on the number of paths to list.
+   *
+   * This may only be called on the driver.
+   *
+   * @param sc                   Spark context used to run parallel listing.
+   * @param paths                Input paths to list
+   * @param hadoopConf           Hadoop configuration
+   * @param filter               Path filter used to exclude leaf files from 
result
+   * @param ignoreMissingFiles   Ignore missing files that happen during 
recursive listing
+   *                             (e.g., due to race conditions)
+   * @param ignoreLocality       Whether to fetch data locality info when 
listing leaf files. If false,
+   *                             this will return `FileStatus` without 
`BlockLocation` info.
+   * @param parallelismThreshold The threshold to enable parallelism. If the 
number of input paths
+   *                             is smaller than this value, this will 
fallback to use
+   *                             sequential listing.
+   * @param parallelismMax       The maximum parallelism for listing. If the 
number of input paths is
+   *                             larger than this value, parallelism will be 
throttled to this value
+   *                             to avoid generating too many tasks.
+   * @return for each input path, the set of discovered files for the path
+   */
+  def parallelListLeafFiles(sc: SparkContext,
+                            paths: Seq[Path],
+                            hadoopConf: Configuration,
+                            filter: PathFilter,
+                            ignoreMissingFiles: Boolean,
+                            ignoreLocality: Boolean,
+                            parallelismThreshold: Int,
+                            parallelismMax: Int): Seq[(Path, Seq[FileStatus])] 
= {
+    parallelListLeafFilesInternal(sc, paths, hadoopConf, filter, isRootLevel = 
true,
+      ignoreMissingFiles, ignoreLocality, parallelismThreshold, parallelismMax)
+  }
+
+  // scalastyle:off parameter.number
+  private def parallelListLeafFilesInternal(sc: SparkContext,
+                                            paths: Seq[Path],
+                                            hadoopConf: Configuration,
+                                            filter: PathFilter,
+                                            isRootLevel: Boolean,
+                                            ignoreMissingFiles: Boolean,
+                                            ignoreLocality: Boolean,
+                                            parallelismThreshold: Int,
+                                            parallelismMax: Int): Seq[(Path, 
Seq[FileStatus])] = {
+
+    // Short-circuits parallel listing when serial listing is likely to be 
faster.
+    if (paths.size <= parallelismThreshold) {
+      // scalastyle:off return
+      return paths.map { path =>
+        val leafFiles = listLeafFiles(
+          path,
+          hadoopConf,
+          filter,
+          Some(sc),
+          ignoreMissingFiles = ignoreMissingFiles,
+          ignoreLocality = ignoreLocality,
+          isRootPath = isRootLevel,
+          parallelismThreshold = parallelismThreshold,
+          parallelismMax = parallelismMax)
+        (path, leafFiles)
+      }
+      // scalastyle:on return
+    }
+
+    logInfo(s"Listing leaf files and directories in parallel under 
${paths.length} paths." +
+      s" The first several paths are: ${paths.take(10).mkString(", ")}.")
+    HiveCatalogMetrics.incrementParallelListingJobCount(1)
+
+    val serializableConfiguration = new SerializableConfiguration(hadoopConf)
+    val serializedPaths = paths.map(_.toString)
+
+    // Set the number of parallelism to prevent following file listing from 
generating many tasks
+    // in case of large #defaultParallelism.
+    val numParallelism = Math.min(paths.size, parallelismMax)
+
+    val previousJobDescription = 
sc.getLocalProperty(SparkContext.SPARK_JOB_DESCRIPTION)
+    val statusMap = try {
+      val description = paths.size match {
+        case 0 =>
+          "Listing leaf files and directories 0 paths"
+        case 1 =>
+          s"Listing leaf files and directories for 1 path:<br/>${paths(0)}"
+        case s =>
+          s"Listing leaf files and directories for $s paths:<br/>${paths(0)}, 
..."
+      }
+      sc.setJobDescription(description)
+      sc
+        .parallelize(serializedPaths, numParallelism)
+        .mapPartitions { pathStrings =>
+          val hadoopConf = serializableConfiguration.value
+          pathStrings.map(new Path(_)).toSeq.map { path =>
+            val leafFiles = listLeafFiles(
+              path = path,
+              hadoopConf = hadoopConf,
+              filter = filter,
+              contextOpt = None, // Can't execute parallel scans on workers
+              ignoreMissingFiles = ignoreMissingFiles,
+              ignoreLocality = ignoreLocality,
+              isRootPath = isRootLevel,
+              parallelismThreshold = Int.MaxValue,
+              parallelismMax = 0)
+            (path, leafFiles)
+          }.iterator
+        }.map { case (path, statuses) =>
+        val serializableStatuses = statuses.map { status =>
+          // Turn FileStatus into SerializableFileStatus so we can send it 
back to the driver
+          val blockLocations = status match {
+            case f: LocatedFileStatus =>
+              f.getBlockLocations.map { loc =>
+                SerializableBlockLocation(
+                  loc.getNames,
+                  loc.getHosts,
+                  loc.getOffset,
+                  loc.getLength)
+              }
+
+            case _ =>
+              Array.empty[SerializableBlockLocation]
+          }
+
+          SerializableFileStatus(
+            status.getPath.toString,
+            status.getLen,
+            status.isDirectory,
+            status.getReplication,
+            status.getBlockSize,
+            status.getModificationTime,
+            status.getAccessTime,
+            blockLocations)
+        }
+        (path.toString, serializableStatuses)
+      }.collect()
+    } finally {
+      sc.setJobDescription(previousJobDescription)
+    }
+
+    // turn SerializableFileStatus back to Status
+    statusMap.map { case (path, serializableStatuses) =>
+      val statuses = serializableStatuses.map { f =>
+        val blockLocations = f.blockLocations.map { loc =>
+          new BlockLocation(loc.names, loc.hosts, loc.offset, loc.length)
+        }
+        new LocatedFileStatus(
+          new FileStatus(
+            f.length, f.isDir, f.blockReplication, f.blockSize, 
f.modificationTime,
+            new Path(f.path)),
+          blockLocations)
+      }
+      (new Path(path), statuses)
+    }
+  }
+  // scalastyle:on parameter.number
+
+  // scalastyle:off parameter.number
+  /**
+   * Lists a single filesystem path recursively. If a `SparkContext` object is 
specified, this
+   * function may launch Spark jobs to parallelize listing based on 
`parallelismThreshold`.
+   *
+   * If sessionOpt is None, this may be called on executors.
+   *
+   * @return all children of path that match the specified filter.
+   */
+  private def listLeafFiles(path: Path,
+                            hadoopConf: Configuration,
+                            filter: PathFilter,
+                            contextOpt: Option[SparkContext],
+                            ignoreMissingFiles: Boolean,
+                            ignoreLocality: Boolean,
+                            isRootPath: Boolean,
+                            parallelismThreshold: Int,
+                            parallelismMax: Int): Seq[FileStatus] = {
+
+    logTrace(s"Listing $path")
+    val fs = path.getFileSystem(hadoopConf)
+
+    // Note that statuses only include FileStatus for the files and dirs 
directly under path,
+    // and does not include anything else recursively.
+    val statuses: Array[FileStatus] = try {
+      fs match {
+        // DistributedFileSystem overrides listLocatedStatus to make 1 single 
call to namenode
+        // to retrieve the file status with the file block location. The 
reason to still fallback
+        // to listStatus is because the default implementation would 
potentially throw a
+        // FileNotFoundException which is better handled by doing the lookups 
manually below.
+        case (_: DistributedFileSystem | _: ViewFileSystem) if !ignoreLocality 
=>
+          val remoteIter = fs.listLocatedStatus(path)
+          new Iterator[LocatedFileStatus]() {
+            def next(): LocatedFileStatus = remoteIter.next
+
+            def hasNext(): Boolean = remoteIter.hasNext
+          }.toArray
+        case _ => fs.listStatus(path)
+      }
+    } catch {
+      // If we are listing a root path for SQL (e.g. a top level directory of 
a table), we need to
+      // ignore FileNotFoundExceptions during this root level of the listing 
because
+      //
+      //  (a) certain code paths might construct an InMemoryFileIndex with 
root paths that
+      //      might not exist (i.e. not all callers are guaranteed to have 
checked
+      //      path existence prior to constructing InMemoryFileIndex) and,
+      //  (b) we need to ignore deleted root paths during REFRESH TABLE, 
otherwise we break
+      //      existing behavior and break the ability drop SessionCatalog 
tables when tables'
+      //      root directories have been deleted (which breaks a number of 
Spark's own tests).
+      //
+      // If we are NOT listing a root path then a FileNotFoundException here 
means that the
+      // directory was present in a previous level of file listing but is 
absent in this
+      // listing, likely indicating a race condition (e.g. concurrent table 
overwrite or S3
+      // list inconsistency).
+      //
+      // The trade-off in supporting existing behaviors / use-cases is that we 
won't be
+      // able to detect race conditions involving root paths being deleted 
during
+      // InMemoryFileIndex construction. However, it's still a net improvement 
to detect and
+      // fail-fast on the non-root cases. For more info see the SPARK-27676 
review discussion.
+      case _: FileNotFoundException if isRootPath || ignoreMissingFiles =>
+        logWarning(s"The directory $path was not found. Was it deleted very 
recently?")
+        Array.empty[FileStatus]
+    }
+
+    val filteredStatuses =
+      statuses.filterNot(status => 
shouldFilterOutPathName(status.getPath.getName))
+
+    val allLeafStatuses = {
+      val (dirs, topLevelFiles) = filteredStatuses.partition(_.isDirectory)
+      val nestedFiles: Seq[FileStatus] = contextOpt match {
+        case Some(context) if dirs.size > parallelismThreshold =>
+          parallelListLeafFilesInternal(
+            context,
+            dirs.map(_.getPath),
+            hadoopConf = hadoopConf,
+            filter = filter,
+            isRootLevel = false,
+            ignoreMissingFiles = ignoreMissingFiles,
+            ignoreLocality = ignoreLocality,
+            parallelismThreshold = parallelismThreshold,
+            parallelismMax = parallelismMax
+          ).flatMap(_._2)
+        case _ =>
+          dirs.flatMap { dir =>
+            listLeafFiles(
+              path = dir.getPath,
+              hadoopConf = hadoopConf,
+              filter = filter,
+              contextOpt = contextOpt,
+              ignoreMissingFiles = ignoreMissingFiles,
+              ignoreLocality = ignoreLocality,
+              isRootPath = false,
+              parallelismThreshold = parallelismThreshold,
+              parallelismMax = parallelismMax)
+          }
+      }
+      val allFiles = topLevelFiles ++ nestedFiles
+      if (filter != null) allFiles.filter(f => filter.accept(f.getPath)) else 
allFiles
+    }
+
+    val missingFiles = mutable.ArrayBuffer.empty[String]
+    val resolvedLeafStatuses = allLeafStatuses.flatMap {
+      case f: LocatedFileStatus =>
+        Some(f)
+
+      // NOTE:
+      //
+      // - Although S3/S3A/S3N file system can be quite slow for remote file 
metadata
+      //   operations, calling `getFileBlockLocations` does no harm here since 
these file system
+      //   implementations don't actually issue RPC for this method.
+      //
+      // - Here we are calling `getFileBlockLocations` in a sequential manner, 
but it should not
+      //   be a big deal since we always use to `parallelListLeafFiles` when 
the number of
+      //   paths exceeds threshold.
+      case f if !ignoreLocality =>
+        // The other constructor of LocatedFileStatus will call 
FileStatus.getPermission(),
+        // which is very slow on some file system (RawLocalFileSystem, which 
is launch a
+        // subprocess and parse the stdout).
+        try {
+          val locations = fs.getFileBlockLocations(f, 0, f.getLen).map { loc =>
+            // Store BlockLocation objects to consume less memory
+            if (loc.getClass == classOf[BlockLocation]) {
+              loc
+            } else {
+              new BlockLocation(loc.getNames, loc.getHosts, loc.getOffset, 
loc.getLength)
+            }
+          }
+          val lfs = new LocatedFileStatus(f.getLen, f.isDirectory, 
f.getReplication, f.getBlockSize,
+            f.getModificationTime, 0, null, null, null, null, f.getPath, 
locations)
+          if (f.isSymlink) {
+            lfs.setSymlink(f.getSymlink)
+          }
+          Some(lfs)
+        } catch {
+          case _: FileNotFoundException if ignoreMissingFiles =>
+            missingFiles += f.getPath.toString
+            None
+        }
+
+      case f => Some(f)
+    }
+
+    if (missingFiles.nonEmpty) {
+      logWarning(
+        s"the following files were missing during file scan:\n  
${missingFiles.mkString("\n  ")}")
+    }
+
+    resolvedLeafStatuses
+  }
+  // scalastyle:on parameter.number
+
+  /** A serializable variant of HDFS's BlockLocation. This is required by 
Hadoop 2.7. */
+  private case class SerializableBlockLocation(names: Array[String],
+                                               hosts: Array[String],
+                                               offset: Long,
+                                               length: Long)
+
+  /** A serializable variant of HDFS's FileStatus. This is required by Hadoop 
2.7. */
+  private case class SerializableFileStatus(path: String,
+                                            length: Long,
+                                            isDir: Boolean,
+                                            blockReplication: Short,
+                                            blockSize: Long,
+                                            modificationTime: Long,
+                                            accessTime: Long,
+                                            blockLocations: 
Array[SerializableBlockLocation])
+
+  /** Checks if we should filter out this path name. */
+  def shouldFilterOutPathName(pathName: String): Boolean = {

Review comment:
       This is the only thing that changed as compared to Spark's 
`HadoopFsUtils`

##########
File path: 
hudi-spark-datasource/hudi-spark-common/src/main/scala/org/apache/hudi/HoodieBaseRelation.scala
##########
@@ -130,22 +158,110 @@ abstract class HoodieBaseRelation(val sqlContext: 
SQLContext,
    * NOTE: DO NOT OVERRIDE THIS METHOD
    */
   override final def buildScan(requiredColumns: Array[String], filters: 
Array[Filter]): RDD[Row] = {
+    // NOTE: In case list of requested columns doesn't contain the Primary Key 
one, we
+    //       have to add it explicitly so that
+    //          - Merging could be performed correctly
+    //          - In case 0 columns are to be fetched (for ex, when doing 
{@code count()} on Spark's [[Dataset]],
+    //          Spark still fetches all the rows to execute the query correctly
+    //
+    //       It's okay to return columns that have not been requested by the 
caller, as those nevertheless will be
+    //       filtered out upstream
+    val fetchedColumns: Array[String] = appendMandatoryColumns(requiredColumns)
+
+    val (requiredAvroSchema, requiredStructSchema) =
+      HoodieSparkUtils.getRequiredSchema(tableAvroSchema, fetchedColumns)
+
+    val filterExpressions = convertToExpressions(filters)
+    val (partitionFilters, dataFilters) = 
filterExpressions.partition(isPartitionPredicate)
+
+    val fileSplits = collectFileSplits(partitionFilters, dataFilters)
+
+    val partitionSchema = StructType(Nil)
+    val tableSchema = HoodieTableSchema(tableStructSchema, 
tableAvroSchema.toString)
+    val requiredSchema = HoodieTableSchema(requiredStructSchema, 
requiredAvroSchema.toString)
+
     // Here we rely on a type erasure, to workaround inherited API restriction 
and pass [[RDD[InternalRow]]] back as [[RDD[Row]]]
     // Please check [[needConversion]] scala-doc for more details
-    doBuildScan(requiredColumns, filters).asInstanceOf[RDD[Row]]
+    composeRDD(fileSplits, partitionSchema, tableSchema, requiredSchema, 
filters).asInstanceOf[RDD[Row]]
   }
 
-  protected def doBuildScan(requiredColumns: Array[String], filters: 
Array[Filter]): RDD[InternalRow]
+  // TODO scala-doc
+  protected def composeRDD(fileSplits: Seq[FileSplit],
+                           partitionSchema: StructType,
+                           tableSchema: HoodieTableSchema,
+                           requiredSchema: HoodieTableSchema,
+                           filters: Array[Filter]): HoodieUnsafeRDD
+
+  // TODO scala-doc
+  protected def collectFileSplits(partitionFilters: Seq[Expression], 
dataFilters: Seq[Expression]): Seq[FileSplit]
+
+  protected def listLatestBaseFiles(globPaths: Seq[Path], partitionFilters: 
Seq[Expression], dataFilters: Seq[Expression]): Map[Path, Seq[FileStatus]] = {
+    if (globPaths.isEmpty) {
+      val partitionDirs = fileIndex.listFiles(partitionFilters, dataFilters)
+      partitionDirs.map(pd => (getPartitionPath(pd.files.head), 
pd.files)).toMap
+    } else {
+      val inMemoryFileIndex = 
HoodieSparkUtils.createInMemoryFileIndex(sparkSession, globPaths)
+      val partitionDirs = inMemoryFileIndex.listFiles(partitionFilters, 
dataFilters)
+
+      val fsView = new HoodieTableFileSystemView(metaClient, timeline, 
partitionDirs.flatMap(_.files).toArray)
+      val latestBaseFiles = 
fsView.getLatestBaseFiles.iterator().asScala.toList.map(_.getFileStatus)
+
+      latestBaseFiles.groupBy(getPartitionPath)
+    }
+  }
+
+  protected def convertToExpressions(filters: Array[Filter]): 
Array[Expression] = {
+    val catalystExpressions = 
HoodieSparkUtils.convertToCatalystExpressions(filters, tableStructSchema)
+
+    val failedExprs = catalystExpressions.zipWithIndex.filter { case (opt, _) 
=> opt.isEmpty }
+    if (failedExprs.nonEmpty) {
+      val failedFilters = failedExprs.map(p => filters(p._2))
+      logWarning(s"Failed to convert Filters into Catalyst expressions 
(${failedFilters.map(_.toString)})")
+    }
+
+    catalystExpressions.filter(_.isDefined).map(_.get).toArray
+  }
+
+  /**
+   * Checks whether given expression only references partition columns
+   * (and involves no sub-query)
+   */
+  protected def isPartitionPredicate(condition: Expression): Boolean = {
+    // Validates that the provided names both resolve to the same entity
+    val resolvedNameEquals = sparkSession.sessionState.analyzer.resolver
+
+    condition.references.forall { r => 
partitionColumns.exists(resolvedNameEquals(r.name, _)) } &&
+      !SubqueryExpression.hasSubquery(condition)
+  }
 
   protected final def appendMandatoryColumns(requestedColumns: Array[String]): 
Array[String] = {
     val missing = mandatoryColumns.filter(col => 
!requestedColumns.contains(col))
     requestedColumns ++ missing
   }
+
+  private def getPrecombineFieldProperty: Option[String] =
+    Option(tableConfig.getPreCombineField)
+      .orElse(optParams.get(DataSourceWriteOptions.PRECOMBINE_FIELD.key)) 
match {
+      // NOTE: This is required to compensate for cases when empty string is 
used to stub
+      //       property value to avoid it being set with the default value
+      // TODO(HUDI-3456) cleanup
+      case Some(f) if !StringUtils.isNullOrEmpty(f) => Some(f)
+      case _ => None
+    }
+
+  private def imbueConfigs(sqlContext: SQLContext): Unit = {
+    
sqlContext.sparkSession.sessionState.conf.setConfString("spark.sql.parquet.filterPushdown",
 "true")
+    
sqlContext.sparkSession.sessionState.conf.setConfString("spark.sql.parquet.recordLevelFilter.enabled",
 "true")
+    
sqlContext.sparkSession.sessionState.conf.setConfString("spark.sql.parquet.enableVectorizedReader",
 "true")
+  }

Review comment:
       Correct. There's no reason to disable vectorization. 
   
   Confirmed this with @YannByron 

##########
File path: 
hudi-spark-datasource/hudi-spark-common/src/main/scala/org/apache/hudi/HoodieBaseRelation.scala
##########
@@ -130,22 +158,110 @@ abstract class HoodieBaseRelation(val sqlContext: 
SQLContext,
    * NOTE: DO NOT OVERRIDE THIS METHOD
    */
   override final def buildScan(requiredColumns: Array[String], filters: 
Array[Filter]): RDD[Row] = {
+    // NOTE: In case list of requested columns doesn't contain the Primary Key 
one, we
+    //       have to add it explicitly so that
+    //          - Merging could be performed correctly
+    //          - In case 0 columns are to be fetched (for ex, when doing 
{@code count()} on Spark's [[Dataset]],
+    //          Spark still fetches all the rows to execute the query correctly
+    //
+    //       It's okay to return columns that have not been requested by the 
caller, as those nevertheless will be
+    //       filtered out upstream
+    val fetchedColumns: Array[String] = appendMandatoryColumns(requiredColumns)
+
+    val (requiredAvroSchema, requiredStructSchema) =
+      HoodieSparkUtils.getRequiredSchema(tableAvroSchema, fetchedColumns)
+
+    val filterExpressions = convertToExpressions(filters)
+    val (partitionFilters, dataFilters) = 
filterExpressions.partition(isPartitionPredicate)
+
+    val fileSplits = collectFileSplits(partitionFilters, dataFilters)
+
+    val partitionSchema = StructType(Nil)
+    val tableSchema = HoodieTableSchema(tableStructSchema, 
tableAvroSchema.toString)
+    val requiredSchema = HoodieTableSchema(requiredStructSchema, 
requiredAvroSchema.toString)
+
     // Here we rely on a type erasure, to workaround inherited API restriction 
and pass [[RDD[InternalRow]]] back as [[RDD[Row]]]
     // Please check [[needConversion]] scala-doc for more details
-    doBuildScan(requiredColumns, filters).asInstanceOf[RDD[Row]]
+    composeRDD(fileSplits, partitionSchema, tableSchema, requiredSchema, 
filters).asInstanceOf[RDD[Row]]
   }
 
-  protected def doBuildScan(requiredColumns: Array[String], filters: 
Array[Filter]): RDD[InternalRow]
+  // TODO scala-doc
+  protected def composeRDD(fileSplits: Seq[FileSplit],
+                           partitionSchema: StructType,
+                           tableSchema: HoodieTableSchema,
+                           requiredSchema: HoodieTableSchema,
+                           filters: Array[Filter]): HoodieUnsafeRDD
+
+  // TODO scala-doc
+  protected def collectFileSplits(partitionFilters: Seq[Expression], 
dataFilters: Seq[Expression]): Seq[FileSplit]
+
+  protected def listLatestBaseFiles(globPaths: Seq[Path], partitionFilters: 
Seq[Expression], dataFilters: Seq[Expression]): Map[Path, Seq[FileStatus]] = {
+    if (globPaths.isEmpty) {
+      val partitionDirs = fileIndex.listFiles(partitionFilters, dataFilters)
+      partitionDirs.map(pd => (getPartitionPath(pd.files.head), 
pd.files)).toMap
+    } else {
+      val inMemoryFileIndex = 
HoodieSparkUtils.createInMemoryFileIndex(sparkSession, globPaths)
+      val partitionDirs = inMemoryFileIndex.listFiles(partitionFilters, 
dataFilters)
+
+      val fsView = new HoodieTableFileSystemView(metaClient, timeline, 
partitionDirs.flatMap(_.files).toArray)
+      val latestBaseFiles = 
fsView.getLatestBaseFiles.iterator().asScala.toList.map(_.getFileStatus)
+
+      latestBaseFiles.groupBy(getPartitionPath)
+    }
+  }
+
+  protected def convertToExpressions(filters: Array[Filter]): 
Array[Expression] = {
+    val catalystExpressions = 
HoodieSparkUtils.convertToCatalystExpressions(filters, tableStructSchema)
+
+    val failedExprs = catalystExpressions.zipWithIndex.filter { case (opt, _) 
=> opt.isEmpty }
+    if (failedExprs.nonEmpty) {
+      val failedFilters = failedExprs.map(p => filters(p._2))
+      logWarning(s"Failed to convert Filters into Catalyst expressions 
(${failedFilters.map(_.toString)})")
+    }
+
+    catalystExpressions.filter(_.isDefined).map(_.get).toArray
+  }
+
+  /**
+   * Checks whether given expression only references partition columns
+   * (and involves no sub-query)
+   */
+  protected def isPartitionPredicate(condition: Expression): Boolean = {
+    // Validates that the provided names both resolve to the same entity
+    val resolvedNameEquals = sparkSession.sessionState.analyzer.resolver
+
+    condition.references.forall { r => 
partitionColumns.exists(resolvedNameEquals(r.name, _)) } &&
+      !SubqueryExpression.hasSubquery(condition)
+  }
 
   protected final def appendMandatoryColumns(requestedColumns: Array[String]): 
Array[String] = {
     val missing = mandatoryColumns.filter(col => 
!requestedColumns.contains(col))
     requestedColumns ++ missing
   }
+
+  private def getPrecombineFieldProperty: Option[String] =
+    Option(tableConfig.getPreCombineField)
+      .orElse(optParams.get(DataSourceWriteOptions.PRECOMBINE_FIELD.key)) 
match {
+      // NOTE: This is required to compensate for cases when empty string is 
used to stub
+      //       property value to avoid it being set with the default value
+      // TODO(HUDI-3456) cleanup
+      case Some(f) if !StringUtils.isNullOrEmpty(f) => Some(f)
+      case _ => None
+    }
+
+  private def imbueConfigs(sqlContext: SQLContext): Unit = {
+    
sqlContext.sparkSession.sessionState.conf.setConfString("spark.sql.parquet.filterPushdown",
 "true")
+    
sqlContext.sparkSession.sessionState.conf.setConfString("spark.sql.parquet.recordLevelFilter.enabled",
 "true")
+    
sqlContext.sparkSession.sessionState.conf.setConfString("spark.sql.parquet.enableVectorizedReader",
 "true")
+  }
 }
 
 object HoodieBaseRelation {
 
-  def isMetadataTable(metaClient: HoodieTableMetaClient) =
+  def getPartitionPath(fileStatus: FileStatus): Path =

Review comment:
       In general yes, but in the context it's scoped for (Relation impl), 
parent of the file -- is the partition path. Or did you have in mind something 
else?




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