Added a pyspark example using tdigest to calculate percentiles for di…

…fferent keys.

pyspark_by_key_example.py

@@ -0,0 +1,25 @@
+from __future__ import print_function
+
+from random import random
+from operator import add 
+from tdigest import TDigest
+
+data = sc.parallelize([(0, random()) for _ in range(1000)] + 
+                      [(1, random() + 1) for _ in range(1000)] + 
+                      [(2, random() + 2) for _ in range(1000)], 10)
+
+def initialise_digest(v):
+    d = TDigest()
+    d.update(v)
+    return d
+
+def update_digest(d, v):
+    d.update(v)
+    return d
+
+percentiles = data\
+    .combineByKey(initialise_digest, update_digest, add)\
+    .map(lambda kv: (kv[0], kv[1].percentile(95)))\
+    .collect()
+
+print(percentiles)