[GitHub] [spark] cloud-fan commented on a diff in pull request #38375: [SPARK-40900][SQL] Reimplement `frequentItems` with dataframe operations

[GitBox <gi...@apache.org>]

cloud-fan commented on code in PR #38375:
URL: https://github.com/apache/spark/pull/38375#discussion_r1004000396


##########
sql/core/src/main/scala/org/apache/spark/sql/execution/stat/FrequentItems.scala:
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@@ -85,42 +51,144 @@ object FrequentItems extends Logging {
       cols: Seq[String],
       support: Double): DataFrame = {
     require(support >= 1e-4 && support <= 1.0, s"Support must be in [1e-4, 1], but got $support.")
-    val numCols = cols.length
+
     // number of max items to keep counts for
     val sizeOfMap = (1 / support).toInt
-    val countMaps = Seq.tabulate(numCols)(i => new FreqItemCounter(sizeOfMap))
-
-    val freqItems = df.select(cols.map(Column(_)) : _*).rdd.treeAggregate(countMaps)(
-      seqOp = (counts, row) => {
-        var i = 0
-        while (i < numCols) {
-          val thisMap = counts(i)
-          val key = row.get(i)
-          thisMap.add(key, 1L)
-          i += 1
-        }
-        counts
-      },
-      combOp = (baseCounts, counts) => {
-        var i = 0
-        while (i < numCols) {
-          baseCounts(i).merge(counts(i))
-          i += 1
+
+    val frequentItemCols = cols.map { col =>
+      val aggExpr = new CollectFrequentItems(functions.col(col).expr, sizeOfMap)
+      Column(aggExpr.toAggregateExpression(isDistinct = false)).as(s"${col}_freqItems")
+    }
+
+    df.select(frequentItemCols: _*)
+  }
+}
+
+case class CollectFrequentItems(
+    child: Expression,
+    size: Int,
+    mutableAggBufferOffset: Int = 0,
+    inputAggBufferOffset: Int = 0) extends TypedImperativeAggregate[mutable.Map[Any, Long]]
+  with ImplicitCastInputTypes with UnaryLike[Expression] {
+  require(size > 0)
+
+  def this(child: Expression, size: Int) = this(child, size, 0, 0)
+
+  // Returns empty array for empty inputs
+  override def nullable: Boolean = false
+
+  override def dataType: DataType = ArrayType(child.dataType, containsNull = child.nullable)
+
+  override def inputTypes: Seq[AbstractDataType] = Seq(AnyDataType)

Review Comment:
   seems we don't have any input type requirement, we don't need to extend `ImplicitCastInputTypes`



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