tdb_sql: Collect stats on wasted UPDATEs in data prop storage.

This will give us better data on which tables will prefer to
SELECT and INSERT and which will prefer to optimistically UPDATE.

r2/r2/lib/db/tdb_sql.py

@@ -508,6 +508,8 @@ def set_data(table, thing_id, **vals):
         i = table.insert(values = dict(thing_id = thing_id))
         i.execute(*inserts)
 
+    return len(inserts)
+
 def incr_data_prop(table, type_id, thing_id, prop, amount):
     t = table
     transactions.add_engine(t.bind)

r2/r2/lib/db/thing.py

@@ -257,7 +257,11 @@ class DataThing(object):
                     data_props[k] = v
 
             if data_props:
-                self._set_data(self._type_id, self._id, **data_props)
+                useless_updates = self._set_data(self._type_id,
+                                                 self._id,
+                                                 **data_props)
+            else:
+                useless_updates = 0
 
             if thing_props:
                 self._set_props(self._type_id, self._id, **thing_props)
@@ -271,6 +275,18 @@ class DataThing(object):
 
             self._cache_myself()
 
+        thing_or_rel = "thing" if self._type_prefix == "t" else "rel"
+
+        if useless_updates:
+            g.stats.simple_event("%s.useless_updates.%s" % (thing_or_rel,
+                                                            self._type_name),
+                                useless_updates)
+        if data_props:
+            g.stats.simple_event("%s.data_props.%s" % (thing_or_rel,
+                                                       self._type_name),
+                                 len(data_props))
+
+
     @classmethod
     def _load_multi(cls, need):
         need = tup(need)