addressed comments v2.0

sql/core/src/main/scala/org/apache/spark/sql/DataFrameStatFunctions.scala

@@ -33,7 +33,8 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou]].
    *
    * @param cols the names of the columns to search frequent items in
-   * @param support The minimum frequency for an item to be considered `frequent`
+   * @param support The minimum frequency for an item to be considered `frequent` Should be greater
+   *                than 1e-4.
    * @return A Local DataFrame with the Array of frequent items for each column.
    */
   def freqItems(cols: Seq[String], support: Double): DataFrame = {
@@ -44,12 +45,39 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * 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]].
-   * Returns items more frequent than 1/1000'th of the time.
+   * Returns items more frequent than 1 percent.
    *
    * @param cols the names of the columns to search frequent items in
    * @return A Local DataFrame with the Array of frequent items for each column.
    */
   def freqItems(cols: Seq[String]): DataFrame = {
-    FrequentItems.singlePassFreqItems(df, cols, 0.001)
+    FrequentItems.singlePassFreqItems(df, cols, 0.01)
+  }
+
+  /**
+   * 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]].
+   *
+   * @param cols the names of the columns to search frequent items in
+   * @param support The minimum frequency for an item to be considered `frequent` Should be greater
+   *                than 1e-4.
+   * @return A Local DataFrame with the Array of frequent items for each column.
+   */
+  def freqItems(cols: List[String], support: Double): DataFrame = {
+    FrequentItems.singlePassFreqItems(df, cols, support)
+  }
+
+  /**
+   * 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]].
+   * Returns items more frequent than 1 percent of the time.
+   *
+   * @param cols the names of the columns to search frequent items in
+   * @return A Local DataFrame with the Array of frequent items for each column.
+   */
+  def freqItems(cols: List[String]): DataFrame = {
+    FrequentItems.singlePassFreqItems(df, cols, 0.01)
   }
 }

sql/core/src/main/scala/org/apache/spark/sql/execution/stat/FrequentItems.scala

@@ -75,16 +75,15 @@ private[sql] object FrequentItems extends Logging {
    *
    * @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`
+   * @param support The minimum frequency for an item to be considered `frequent`. Should be greater
+   *                than 1e-4.
    * @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) {
-      logWarning(s"The selected support ($support) is too small, and might cause memory problems.")
-    }
+    require(support >= 1e-4, s"support ($support) must be greater than 1e-4.")
     val numCols = cols.length
     // number of max items to keep counts for
     val sizeOfMap = (1 / support).toInt

sql/core/src/test/java/test/org/apache/spark/sql/JavaDataFrameSuite.java

@@ -22,10 +22,7 @@
 
 import org.apache.spark.api.java.JavaRDD;
 import org.apache.spark.api.java.JavaSparkContext;
-import org.apache.spark.sql.DataFrame;
-import org.apache.spark.sql.Row;
-import org.apache.spark.sql.SQLContext;
-import org.apache.spark.sql.TestData$;
+import org.apache.spark.sql.*;
 import org.apache.spark.sql.test.TestSQLContext;
 import org.apache.spark.sql.test.TestSQLContext$;
 import org.apache.spark.sql.types.*;
@@ -36,6 +33,7 @@
 import scala.collection.mutable.Buffer;
 
 import java.io.Serializable;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
@@ -178,4 +176,13 @@ public void testCreateDataFrameFromJavaBeans() {
       Assert.assertEquals(bean.getD().get(i), d.apply(i));
     }
   }
+
+  @Test
+  public void testFrequentItems() {
+    DataFrame df = context.table("testData2");
+    List<String> cols = Arrays.asList("a");
+    DataFrame results = df.stat().freqItems(JavaConversions.asScalaIterable(cols).toList(), 0.2);
+    System.out.println(results.collect()[0].getSeq(0));
+    Assert.assertTrue(results.collect()[0].getSeq(0).contains(1));
+  }
 }

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

@@ -17,27 +17,31 @@
 
 package org.apache.spark.sql
 
-import org.apache.spark.sql.test.TestSQLContext
-import org.apache.spark.sql.types._
 import org.scalatest.FunSuite
+import org.scalatest.Matchers._
+
+import org.apache.spark.sql.test.TestSQLContext
+import org.apache.spark.sql.test.TestSQLContext.implicits._
 
 class DataFrameStatSuite extends FunSuite  {
 
   val sqlCtx = TestSQLContext
 
   test("Frequent Items") {
     def toLetter(i: Int): String = (i + 96).toChar.toString
-    val rows = Array.tabulate(1000)(i => if (i % 3 == 0) (1, toLetter(1)) else (i, toLetter(i)))
-    val rowRdd = sqlCtx.sparkContext.parallelize(rows.map(v => Row(v._1, v._2)))
-    val schema = StructType(StructField("numbers", IntegerType, false) ::
-                            StructField("letters", StringType, false) :: Nil)
-    val df = sqlCtx.createDataFrame(rowRdd, schema)
+    val rows = Array.tabulate(1000) { i =>
+      if (i % 3 == 0) (1, toLetter(1), -1.0) else (i, toLetter(i), i * -1.0)
+    }
+    val df = sqlCtx.sparkContext.parallelize(rows).toDF("numbers", "letters", "negDoubles")
 
     val results = df.stat.freqItems(Array("numbers", "letters"), 0.1)
     val items = results.collect().head
-    assert(items.getSeq(0).contains(1),
-      "1 should be the frequent item for column 'numbers")
-    assert(items.getSeq(1).contains(toLetter(1)),
-      s"${toLetter(1)} should be the frequent item for column 'letters'")
+    items.getSeq[Int](0) should contain (1)
+    items.getSeq[String](1) should contain (toLetter(1))
+
+    val singleColResults = df.stat.freqItems(Array("negDoubles"), 0.1)
+    val items2 = singleColResults.collect().head
+    items2.getSeq[Double](0) should contain (-1.0)
+
   }
 }