Merge pull request #124 from CartoDB/add-kurtosis

Add kurtosis and skewness

test/CDB_StatsTest.sql

@@ -0,0 +1,16 @@
+-- continuous uniform distribution has kurtosis = -6/5, skewness = 0.0
+-- http://mathworld.wolfram.com/UniformDistribution.html
+set client_min_messages to ERROR;
+
+With dist As (
+  SELECT random()::numeric As val
+  FROM generate_series(1,50000) t
+)
+
+SELECT 
+  -- does random dist values match within 1% of known values
+  abs(CDB_Kurtosis(array_agg(val)) + 1.20) < 1e-2 As kurtosis,
+  abs(CDB_Skewness(array_agg(val)) - 0) < 1e-2 As skewness
+FROM dist;
+
+set client_min_messages to NOTICE;

test/CDB_StatsTest_expect

@@ -0,0 +1,3 @@
+SET
+t|t
+SET