Improve GpuExpand by pre-projecting some columns (#25)

Some rules in Spark will put non-leaf expressions into Expand projections,
then it can not leverage the GPU tiered projection across the projection lists.

So this PR tries to factor out these expressions and evaluate them before
expanding to avoid duplicate evaluation for semantic-equal (sub) expressions.

---------

Signed-off-by: Firestarman <firestarmanllc@gmail.com>

sql-plugin/src/main/scala/com/nvidia/spark/rapids/GpuExpandExec.scala

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020-2023, NVIDIA CORPORATION.
+ * Copyright (c) 2020-2024, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,6 +15,9 @@
  */
 package com.nvidia.spark.rapids
 
+import scala.collection.mutable
+import scala.util.Random
+
 import ai.rapids.cudf.NvtxColor
 import com.nvidia.spark.rapids.Arm.withResource
 import com.nvidia.spark.rapids.GpuMetric._
@@ -76,6 +79,7 @@ case class GpuExpandExec(
   override val outputBatchesLevel: MetricsLevel = MODERATE_LEVEL
   override lazy val additionalMetrics: Map[String, GpuMetric] = Map(
     OP_TIME -> createNanoTimingMetric(MODERATE_LEVEL, DESCRIPTION_OP_TIME),
+    "preprojectTime" -> createNanoTimingMetric(MODERATE_LEVEL, "pre-projection time"),
     NUM_INPUT_ROWS -> createMetric(DEBUG_LEVEL, DESCRIPTION_NUM_INPUT_ROWS),
     NUM_INPUT_BATCHES -> createMetric(DEBUG_LEVEL, DESCRIPTION_NUM_INPUT_BATCHES))
 
@@ -88,22 +92,86 @@ case class GpuExpandExec(
     AttributeSet(projections.flatten.flatMap(_.references))
 
   override protected def internalDoExecuteColumnar(): RDD[ColumnarBatch] = {
-    val boundProjections = projections.map { pl =>
-      GpuBindReferences.bindGpuReferencesTiered(pl, child.output, useTieredProject)
-    }
-
     // cache in a local to avoid serializing the plan
     val metricsMap = allMetrics
 
+    val notAllLeaf = preprojectionList.exists(_.children.nonEmpty)
+    val (boundProjections, preprojectIter) = if (useTieredProject && notAllLeaf) {
+      // Got some complicated expressions and tiered projection is enabled.
+      // Then try to do the pre-projection first.
+      val boundPreprojections = GpuBindReferences.bindGpuReferencesTiered(
+        preprojectionList, child.output, useTieredProject)
+      val preprojectIterFunc: Iterator[ColumnarBatch] => Iterator[ColumnarBatch] = iter =>
+        iter.map{ cb =>
+          val start = System.nanoTime()
+          val ret = boundPreprojections.projectAndCloseWithRetrySingleBatch(
+            SpillableColumnarBatch(cb, SpillPriorities.ACTIVE_ON_DECK_PRIORITY))
+          val timeTaken = System.nanoTime() - start
+          metricsMap("preprojectTime") += timeTaken
+          metricsMap(OP_TIME) += timeTaken
+          ret
+        }
+      val preprojectAttrs = preprojectionList.map(_.toAttribute)
+      val boundLists = updatedProjections.map { pl =>
+        GpuBindReferences.bindGpuReferencesTiered(pl, preprojectAttrs, useTieredProject)
+      }
+      (boundLists, preprojectIterFunc)
+    } else {
+      val boundLists = projections.map { pl =>
+        GpuBindReferences.bindGpuReferencesTiered(pl, child.output, useTieredProject)
+      }
+      (boundLists, identity[Iterator[ColumnarBatch]] _)
+    }
+
     child.executeColumnar().mapPartitions { it =>
-      new GpuExpandIterator(boundProjections, metricsMap, it)
+      new GpuExpandIterator(boundProjections, metricsMap, preprojectIter(it))
     }
   }
 
   override protected def doExecute(): RDD[InternalRow] = {
     throw new IllegalStateException("ROW BASED PROCESSING IS NOT SUPPORTED")
   }
 
+  /**
+   * Get the expressions that need to be pre-projected, along with the updated
+   * projections for expanding.
+   *
+   * Some rules (e.g. RewriteDistinctAggregates) in Spark will put non-leaf expressions
+   * in Expand projections, then it can not leverage the GPU tiered projection across
+   * the projection lists.
+   * So here tries to factor out these expressions for later evaluations before
+   * expanding to avoid duplicate evaluation for semantic-equal (sub) expressions.
+   */
+  private[this] lazy val (preprojectionList, updatedProjections) = {
+    val projectListBuffer = mutable.Set[NamedExpression]()
+    val newProjections = projections.map { proList =>
+      proList.map {
+        case attr: AttributeReference if child.outputSet.contains(attr) =>
+          // A ref to child output, add it to pre-projection for passthrough.
+          projectListBuffer += attr
+          attr
+        case leaf if leaf.children.isEmpty =>
+          // A leaf expression is simple enough, not necessary for pre-projection.
+          // e.g. GpuLiteral, and two internal columns (grouping id and grouping
+          // position) specific to Expand.
+          leaf
+        case notLeafNamed: NamedExpression =>
+          logWarning(s"==>Got a named non-leaf expression: $notLeafNamed for preprojection")
+          // A named expression, e.g. GpuAlias. Add it for pre-projection.
+          projectListBuffer += notLeafNamed
+          // Replace with its reference
+          notLeafNamed.toAttribute
+        case notLeaf =>
+          // Wrap by a GpuAlias
+          logWarning(s"==>Got a non-leaf expression: $notLeaf for preprojection")
+          val alias = GpuAlias(notLeaf, s"_preproject-c${Random.nextInt}")()
+          projectListBuffer += alias
+          // Replace with the reference of the new GpuAlias.
+          alias.toAttribute
+      }
+    }
+    (projectListBuffer.toList, newProjections)
+  }
 }
 
 class GpuExpandIterator(