Gather usage metrics to db table with sql trigger

resources/migrations/20170317210604-user-stats.up.sql

@@ -0,0 +1,23 @@
+
+drop table usage_metrics;
+CREATE TABLE usage_metrics (
+    registered_users int,
+    users_with_address int,
+    change_timestamp timestamp without time zone
+ DEFAULT timezone('utc'::text, now()));
+
+
+CREATE OR REPLACE FUNCTION store_usage_metrics() RETURNS TRIGGER AS $usage_metrics$
+BEGIN
+  insert into usage_metrics (registered_users, users_with_address)
+  values ((select count(*) from users),
+          (select count(*) from users
+          where address is not null));
+
+  return null;
+END;
+$usage_metrics$ LANGUAGE plpgsql;
+
+CREATE TRIGGER usage_metrics
+AFTER INSERT OR UPDATE OR DELETE ON users
+execute PROCEDURE store_usage_metrics();