[SPARK-47836][SQL] Use doubles sketch replace the GK algorithm for approximate quantile computation, significantly improving merge performance

Author: lifulong-zhihu

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/ApproximatePercentile.scala

@@ -17,9 +17,8 @@
 
 package org.apache.spark.sql.catalyst.expressions.aggregate
 
-import java.nio.ByteBuffer
-
-import com.google.common.primitives.{Doubles, Ints, Longs}
+import org.apache.datasketches.memory.Memory
+import org.apache.datasketches.quantiles.{DoublesSketch, DoublesUnion, UpdateDoublesSketch}
 
 import org.apache.spark.SparkException
 import org.apache.spark.sql.catalyst.InternalRow
@@ -31,10 +30,7 @@ import org.apache.spark.sql.catalyst.expressions.aggregate.ApproximatePercentile
 import org.apache.spark.sql.catalyst.trees.TernaryLike
 import org.apache.spark.sql.catalyst.types.PhysicalNumericType
 import org.apache.spark.sql.catalyst.util.{ArrayData, GenericArrayData}
-import org.apache.spark.sql.catalyst.util.QuantileSummaries
-import org.apache.spark.sql.catalyst.util.QuantileSummaries.{defaultCompressThreshold, Stats}
 import org.apache.spark.sql.types._
-import org.apache.spark.util.ArrayImplicits._
 
 /**
  * The ApproximatePercentile function returns the approximate percentile(s) of a column at the given
@@ -267,35 +263,40 @@ object ApproximatePercentile {
   // The default relative error can be deduced by defaultError = 1.0 / DEFAULT_PERCENTILE_ACCURACY
   val DEFAULT_PERCENTILE_ACCURACY: Int = 10000
 
+  def nextPowOf2(relativeError: Double): Int = {
+    val baseK = DoublesSketch.getKFromEpsilon(relativeError, true)
+    if (baseK == 1 || (baseK & (baseK - 1)) == 0) {
+      baseK
+    } else {
+      Integer.highestOneBit(baseK) * 2
+    }
+  }
+
   /**
    * PercentileDigest is a probabilistic data structure used for approximating percentiles
-   * with limited memory. PercentileDigest is backed by [[QuantileSummaries]].
+   * with limited memory. PercentileDigest is backed by [[DoublesSketch]].
    *
-   * @param summaries underlying probabilistic data structure [[QuantileSummaries]].
+   * @param sketch underlying probabilistic data structure [[DoublesSketch]].
    */
-  class PercentileDigest(private var summaries: QuantileSummaries) {
+  class PercentileDigest(private var sketch: UpdateDoublesSketch) {
 
     def this(relativeError: Double) = {
-      this(new QuantileSummaries(defaultCompressThreshold, relativeError, compressed = true))
+      this(DoublesSketch.builder().setK(ApproximatePercentile.nextPowOf2(relativeError)).build())
     }
 
-    private[sql] def isCompressed: Boolean = summaries.compressed
-
-    /** Returns compressed object of [[QuantileSummaries]] */
-    def quantileSummaries: QuantileSummaries = {
-      if (!isCompressed) compress()
-      summaries
-    }
+    def sketchInfo: UpdateDoublesSketch = sketch
 
     /** Insert an observation value into the PercentileDigest data structure. */
     def add(value: Double): Unit = {
-      summaries = summaries.insert(value)
+      sketch.update(value)
     }
 
     /** In-place merges in another PercentileDigest. */
     def merge(other: PercentileDigest): Unit = {
-      if (!isCompressed) compress()
-      summaries = summaries.merge(other.quantileSummaries)
+      val doublesUnion = DoublesUnion.builder().setMaxK(sketch.getK).build()
+      doublesUnion.union(sketch)
+      doublesUnion.union(other.sketch)
+      sketch = doublesUnion.getResult
     }
 
     /**
@@ -309,17 +310,12 @@ object ApproximatePercentile {
      * }}}
      */
     def getPercentiles(percentages: Array[Double]): Seq[Double] = {
-      if (!isCompressed) compress()
-      if (summaries.count == 0 || percentages.length == 0) {
-        Array.emptyDoubleArray.toImmutableArraySeq
+      if (!sketch.isEmpty) {
+        sketch.getQuantiles(percentages).toSeq
       } else {
-        summaries.query(percentages.toImmutableArraySeq).get
+        Seq.empty[Double]
       }
     }
-
-    private final def compress(): Unit = {
-      summaries = summaries.compress()
-    }
   }
 
   /**
@@ -329,52 +325,14 @@ object ApproximatePercentile {
    */
   class PercentileDigestSerializer {
 
-    private final def length(summaries: QuantileSummaries): Int = {
-      // summaries.compressThreshold, summary.relativeError, summary.count
-      Ints.BYTES + Doubles.BYTES + Longs.BYTES +
-      // length of summary.sampled
-      Ints.BYTES +
-      // summary.sampled, Array[Stat(value: Double, g: Long, delta: Long)]
-      summaries.sampled.length * (Doubles.BYTES + Longs.BYTES + Longs.BYTES)
-    }
-
     final def serialize(obj: PercentileDigest): Array[Byte] = {
-      val summary = obj.quantileSummaries
-      val buffer = ByteBuffer.wrap(new Array(length(summary)))
-      buffer.putInt(summary.compressThreshold)
-      buffer.putDouble(summary.relativeError)
-      buffer.putLong(summary.count)
-      buffer.putInt(summary.sampled.length)
-
-      var i = 0
-      while (i < summary.sampled.length) {
-        val stat = summary.sampled(i)
-        buffer.putDouble(stat.value)
-        buffer.putLong(stat.g)
-        buffer.putLong(stat.delta)
-        i += 1
-      }
-      buffer.array()
+      val sketch = obj.sketchInfo
+      sketch.toByteArray(false)
     }
 
     final def deserialize(bytes: Array[Byte]): PercentileDigest = {
-      val buffer = ByteBuffer.wrap(bytes)
-      val compressThreshold = buffer.getInt()
-      val relativeError = buffer.getDouble()
-      val count = buffer.getLong()
-      val sampledLength = buffer.getInt()
-      val sampled = new Array[Stats](sampledLength)
-
-      var i = 0
-      while (i < sampledLength) {
-        val value = buffer.getDouble()
-        val g = buffer.getLong()
-        val delta = buffer.getLong()
-        sampled(i) = Stats(value, g, delta)
-        i += 1
-      }
-      val summary = new QuantileSummaries(compressThreshold, relativeError, sampled, count, true)
-      new PercentileDigest(summary)
+      val sketch = DoublesSketch.heapify(Memory.wrap(bytes))
+      new PercentileDigest(sketch.asInstanceOf[UpdateDoublesSketch])
     }
   }
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/aggregate/ApproximatePercentileSuite.scala

@@ -426,12 +426,18 @@ class ApproximatePercentileSuite extends SparkFunSuite {
   }
 
   private def compareEquals(left: PercentileDigest, right: PercentileDigest): Boolean = {
-    val leftSummary = left.quantileSummaries
-    val rightSummary = right.quantileSummaries
-    leftSummary.compressThreshold == rightSummary.compressThreshold &&
-      leftSummary.relativeError == rightSummary.relativeError &&
-      leftSummary.count == rightSummary.count &&
-      leftSummary.sampled.sameElements(rightSummary.sampled)
+    val leftSketch = left.sketchInfo
+    val rightSketch = right.sketchInfo
+    if (leftSketch.isEmpty && rightSketch.isEmpty) {
+      true
+    } else if (leftSketch.isEmpty || rightSketch.isEmpty) {
+      false
+    } else {
+      leftSketch.getK == rightSketch.getK &&
+        leftSketch.getMaxItem == rightSketch.getMaxItem &&
+        leftSketch.getMinItem == rightSketch.getMinItem &&
+        leftSketch.getN == rightSketch.getN
+    }
   }
 
   private def assertEqual[T](left: T, right: T): Unit = {

sql/core/src/test/scala/org/apache/spark/sql/ApproximatePercentileQuerySuite.scala

@@ -20,7 +20,6 @@ package org.apache.spark.sql
 import java.sql.{Date, Timestamp}
 import java.time.{Duration, LocalDateTime, Period}
 
-import org.apache.spark.sql.catalyst.expressions.aggregate.ApproximatePercentile
 import org.apache.spark.sql.catalyst.expressions.aggregate.ApproximatePercentile.DEFAULT_PERCENTILE_ACCURACY
 import org.apache.spark.sql.catalyst.expressions.aggregate.ApproximatePercentile.PercentileDigest
 import org.apache.spark.sql.catalyst.util.DateTimeUtils
@@ -291,18 +290,6 @@ class ApproximatePercentileQuerySuite extends QueryTest with SharedSparkSession
     }
   }
 
-  test("SPARK-24013: unneeded compress can cause performance issues with sorted input") {
-    val buffer = new PercentileDigest(1.0D / ApproximatePercentile.DEFAULT_PERCENTILE_ACCURACY)
-    var compressCounts = 0
-    (1 to 10000000).foreach { i =>
-      buffer.add(i)
-      if (buffer.isCompressed) compressCounts += 1
-    }
-    assert(compressCounts > 0)
-    buffer.quantileSummaries
-    assert(buffer.isCompressed)
-  }
-
   test("SPARK-32908: maximum target error in percentile_approx") {
     withTempView(table) {
       spark.read