[IMP] base: performances of view._check_xml

View validation accounts for a fair cost of module installation, most of
that is spent checking for view validity, and a significant fraction
of *that* is spent reading the validated view from the DB.

Turns out _check_xml requested *all* view fields even though it needed
none of them save for the arch. Specifying fields to read_combined
rather than fetching all fields seems to result in a ~30% speedup of
_check_xml (under line_profiler). Most of the rest is spent fetching
sub-views (in get_inheriting_view_arch), I don't know that it can be
improved without hand-crafting a fairly complex SQL request.

openerp/addons/base/ir/ir_ui_view.py

@@ -203,7 +203,7 @@ class view(osv.osv):
         # Sanity checks: the view should not break anything upon rendering!
         # Any exception raised below will cause a transaction rollback.
         for view in self.browse(cr, uid, ids, context):
-            view_def = self.read_combined(cr, uid, view.id, None, context=context)
+            view_def = self.read_combined(cr, uid, view.id, ['arch'], context=context)
             view_arch_utf8 = view_def['arch']
             if view.type != 'qweb':
                 view_doc = etree.fromstring(view_arch_utf8)
@@ -527,7 +527,7 @@ class view(osv.osv):
 
         # arch and model fields are always returned
         if fields:
-            fields = list(set(fields) | set(['arch', 'model']))
+            fields = list({'arch', 'model'}.union(fields))
 
         # read the view arch
         [view] = self.read(cr, uid, [root_id], fields=fields, context=context)