[SPARK-5865][API DOC] Add doc warnings for methods that return local data structures

rxin srowen
I work out note message for rdd.take function, please help to review.

If it's fine, I can apply to all other function later.

Author: Tommy YU <tummyyu@163.com>

Closes #10874 from Wenpei/spark-5865-add-warning-for-localdatastructure.

1 files changed, 6 insertions, 0 deletions

diff --git a/python/pyspark/sql/dataframe.py b/python/pyspark/sql/dataframe.py
index 90a6b5d9c0..3a8c8305ee 100644
--- a/python/pyspark/sql/dataframe.py
+++ b/python/pyspark/sql/dataframe.py
@@ -739,6 +739,9 @@ class DataFrame(object):
     def head(self, n=None):
         """Returns the first ``n`` rows.
 
+        Note that this method should only be used if the resulting array is expected
+        to be small, as all the data is loaded into the driver's memory.
+
         :param n: int, default 1. Number of rows to return.
         :return: If n is greater than 1, return a list of :class:`Row`.
             If n is 1, return a single Row.
@@ -1330,6 +1333,9 @@ class DataFrame(object):
     def toPandas(self):
         """Returns the contents of this :class:`DataFrame` as Pandas ``pandas.DataFrame``.
 
+        Note that this method should only be used if the resulting Pandas's DataFrame is expected
+        to be small, as all the data is loaded into the driver's memory.
+
         This is only available if Pandas is installed and available.
 
         >>> df.toPandas()  # doctest: +SKIP