Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/5799#discussion_r29406990
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/stat/FrequentItems.scala
---
@@ -0,0 +1,127 @@
+/*
+* 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.stat
+
+import org.apache.spark.Logging
+import org.apache.spark.sql.{Column, DataFrame, Row}
+import org.apache.spark.sql.catalyst.plans.logical.LocalRelation
+import org.apache.spark.sql.types.{ArrayType, StructField, StructType}
+
+import scala.collection.mutable.{Map => MutableMap}
+
+private[sql] object FrequentItems extends Logging {
+
+ /** A helper class wrapping `MutableMap[Any, Long]` for simplicity. */
+ private class FreqItemCounter(size: Int) extends Serializable {
+ val baseMap: MutableMap[Any, Long] = MutableMap.empty[Any, Long]
+
+ /**
+ * Add a new example to the counts if it exists, otherwise deduct the
count
+ * from existing items.
+ */
+ def add(key: Any, count: Long): this.type = {
+ if (baseMap.contains(key)) {
+ baseMap(key) += count
+ } else {
+ if (baseMap.size < size) {
+ baseMap += key -> count
+ } else {
+ // TODO: Make this more efficient... A flatMap?
+ baseMap.retain((k, v) => v > count)
+ baseMap.transform((k, v) => v - count)
+ }
+ }
+ this
+ }
+
+ /**
+ * Merge two maps of counts.
+ * @param other The map containing the counts for that partition
+ */
+ def merge(other: FreqItemCounter): this.type = {
+ other.toSeq.foreach { case (k, v) =>
+ add(k, v)
+ }
+ this
+ }
+
+ def toSeq: Seq[(Any, Long)] = baseMap.toSeq
+
+ def foldLeft[A, B](start: A)(f: (A, (Any, Long)) => A): A =
baseMap.foldLeft(start)(f)
+
+ def freqItems: Seq[Any] = baseMap.keys.toSeq
+ }
+
+ /**
+ * Finding frequent items for columns, possibly with false positives.
Using the
+ * frequent element count algorithm described in
+ * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp,
Schenker, and Papadimitriou]].
+ * For Internal use only.
+ *
+ * @param df The input DataFrame
+ * @param cols the names of the columns to search frequent items in
+ * @param support The minimum frequency for an item to be considered
`frequent`
+ * @return A Local DataFrame with the Array of frequent items for each
column.
+ */
+ private[sql] def singlePassFreqItems(
+ df: DataFrame,
+ cols: Seq[String],
+ support: Double): DataFrame = {
+ if (support < 1e-6) {
--- End diff --
might as well do e-5 since e-6 can be super large and not very practical.
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