[minor doc] Add exploratory data analysis warning for DataFrame.stat.freqItem API

Author: Reynold Xin <rxin@databricks.com>

Closes #6569 from rxin/freqItemsWarning and squashes the following commits:

7eec145 [Reynold Xin] [minor doc] Add exploratory data analysis warning for DataFrame.stat.freqItem API.

1 files changed, 12 insertions, 0 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/DataFrameStatFunctions.scala b/sql/core/src/main/scala/org/apache/spark/sql/DataFrameStatFunctions.scala
index b624eaa201..edb9ed7bba 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/DataFrameStatFunctions.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/DataFrameStatFunctions.scala
@@ -97,6 +97,9 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou]].
    * The `support` should be greater than 1e-4.
    *
+   * This function is meant for exploratory data analysis, as we make no guarantee about the
+   * backward compatibility of the schema of the resulting [[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`. Should be greater
    *                than 1e-4.
@@ -114,6 +117,9 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou]].
    * Uses a `default` support of 1%.
    *
+   * This function is meant for exploratory data analysis, as we make no guarantee about the
+   * backward compatibility of the schema of the resulting [[DataFrame]].
+   *
    * @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.
    *
@@ -128,6 +134,9 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * frequent element count algorithm described in
    * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou]].
    *
+   * This function is meant for exploratory data analysis, as we make no guarantee about the
+   * backward compatibility of the schema of the resulting [[DataFrame]].
+   *
    * @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.
    *
@@ -143,6 +152,9 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
    * [[http://dx.doi.org/10.1145/762471.762473, proposed by Karp, Schenker, and Papadimitriou]].
    * Uses a `default` support of 1%.
    *
+   * This function is meant for exploratory data analysis, as we make no guarantee about the
+   * backward compatibility of the schema of the resulting [[DataFrame]].
+   *
    * @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.
    *