tdb_sql: Optimistically do UPDATES and fall back to INSERTS.

Doing a SELECT on the data table to determine which keys already
exist was adding too much extra load to the Account master. This
attempts to alleviate that by optimistically trying an UPDATE and
if no rows were actually changed by the UPDATE, doing an INSERT
instead.

r2/r2/lib/db/tdb_sql.py

@@ -491,12 +491,8 @@ def db2py(val, kind):
 
 #TODO i don't need type_id
 def set_data(table, type_id, thing_id, **vals):
-    s = sa.select([table.c.key], sa.and_(table.c.thing_id == thing_id))
-
     transactions.add_engine(table.bind)
-    keys = [x.key for x in s.execute().fetchall()]
 
-    i = table.insert(values = dict(thing_id = thing_id))
     u = table.update(sa.and_(table.c.thing_id == thing_id,
                              table.c.key == sa.bindparam('_key')))
 
@@ -504,14 +500,13 @@ def set_data(table, type_id, thing_id, **vals):
     for key, val in vals.iteritems():
         val, kind = py2db(val, return_kind=True)
 
-        #TODO one update?
-        if key in keys:
-            u.execute(_key = key, value = val, kind = kind)
-        else:
+        uresult = u.execute(_key = key, value = val, kind = kind)
+        if not uresult.rowcount:
             inserts.append({'key':key, 'value':val, 'kind': kind})
 
     #do one insert
     if inserts:
+        i = table.insert(values = dict(thing_id = thing_id))
         i.execute(*inserts)
 
 def incr_data_prop(table, type_id, thing_id, prop, amount):