[SPARK-13479][SQL][PYTHON] Added Python API for approxQuantile

## What changes were proposed in this pull request?

* Scala DataFrameStatFunctions: Added version of approxQuantile taking a List instead of an Array, for Python compatbility
* Python DataFrame and DataFrameStatFunctions: Added approxQuantile

## How was this patch tested?

* unit test in sql/tests.py

Documentation was copied from the existing approxQuantile exactly.

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #11356 from jkbradley/approx-quantile-python.

1 files changed, 10 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 39a31ab028..3eb1f0f0d5 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
@@ -71,6 +71,16 @@ final class DataFrameStatFunctions private[sql](df: DataFrame) {
   }
 
   /**
+   * Python-friendly version of [[approxQuantile()]]
+   */
+  private[spark] def approxQuantile(
+      col: String,
+      probabilities: List[Double],
+      relativeError: Double): java.util.List[Double] = {
+    approxQuantile(col, probabilities.toArray, relativeError).toList.asJava
+  }
+
+  /**
    * Calculate the sample covariance of two numerical columns of a DataFrame.
    * @param col1 the name of the first column
    * @param col2 the name of the second column