[SPARK-7548] [SQL] Add explode function for DataFrames

Add an `explode` function for dataframes and modify the analyzer so that single table generating functions can be present in a select clause along with other expressions.   There are currently the following restrictions:
 - only top level TGFs are allowed (i.e. no `select(explode('list) + 1)`)
 - only one may be present in a single select to avoid potentially confusing implicit Cartesian products.

TODO:
 - [ ] Python

Author: Michael Armbrust <michael@databricks.com>

Closes #6107 from marmbrus/explodeFunction and squashes the following commits:

7ee2c87 [Michael Armbrust] whitespace
6f80ba3 [Michael Armbrust] Update dataframe.py
c176c89 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into explodeFunction
81b5da3 [Michael Armbrust] style
d3faa05 [Michael Armbrust] fix self join case
f9e1e3e [Michael Armbrust] fix python, add since
4f0d0a9 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into explodeFunction
e710fe4 [Michael Armbrust] add java and python
52ca0dc [Michael Armbrust] [SPARK-7548][SQL] Add explode function for dataframes.

1 files changed, 9 insertions, 3 deletions

diff --git a/python/pyspark/sql/dataframe.py b/python/pyspark/sql/dataframe.py
index 82cb1c2fdb..2ed95ac8e2 100644
--- a/python/pyspark/sql/dataframe.py
+++ b/python/pyspark/sql/dataframe.py
@@ -1511,13 +1511,19 @@ class Column(object):
     isNull = _unary_op("isNull", "True if the current expression is null.")
     isNotNull = _unary_op("isNotNull", "True if the current expression is not null.")
 
-    def alias(self, alias):
-        """Return a alias for this column
+    def alias(self, *alias):
+        """Returns this column aliased with a new name or names (in the case of expressions that
+        return more than one column, such as explode).
 
         >>> df.select(df.age.alias("age2")).collect()
         [Row(age2=2), Row(age2=5)]
         """
-        return Column(getattr(self._jc, "as")(alias))
+
+        if len(alias) == 1:
+            return Column(getattr(self._jc, "as")(alias[0]))
+        else:
+            sc = SparkContext._active_spark_context
+            return Column(getattr(self._jc, "as")(_to_seq(sc, list(alias))))
 
     @ignore_unicode_prefix
     def cast(self, dataType):