Don't post-SELECT columns w/o a server default/onupdate for eager_defaults

Fixed a major inefficiency in the "eager_defaults" feature whereby
an unnecessary SELECT would be emitted for column values where the
ORM had explicitly inserted NULL, corresponding to attributes that
were unset on the object but did not have any server default
specified, as well as expired attributes on update that nevertheless
had no server onupdate set up.   As these columns are not part of the
RETURNING that eager_defaults tries to use, they should not be
post-SELECTed either.

Change-Id: I0d4f1e9d3d9717d68dcc0592f69456a1f1c36df8
Fixes: #3909

2 files changed, 22 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index f45b56ae7..962486d58 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -2042,6 +2042,22 @@ class Mapper(InspectionAttr):
         )
 
     @_memoized_configured_property
+    def _server_default_plus_onupdate_propkeys(self):
+        result = set()
+
+        for table, columns in self._cols_by_table.items():
+            for col in columns:
+                if (
+                        (
+                            col.server_default is not None or
+                            col.server_onupdate is not None
+                        ) and col in self._columntoproperty
+                ):
+                    result.add(self._columntoproperty[col].key)
+
+        return result
+
+    @_memoized_configured_property
     def _server_onupdate_default_cols(self):
         return dict(
             (
diff --git a/lib/sqlalchemy/orm/persistence.py b/lib/sqlalchemy/orm/persistence.py
index 8f3e5de58..4d1e38d3f 100644
--- a/lib/sqlalchemy/orm/persistence.py
+++ b/lib/sqlalchemy/orm/persistence.py
@@ -994,8 +994,12 @@ def _finalize_insert_update_commands(base_mapper, uowtransaction, states):
         toload_now = []
 
         if base_mapper.eager_defaults:
-            toload_now.extend(state._unloaded_non_object)
-        elif mapper.version_id_col is not None and \
+            toload_now.extend(
+                state._unloaded_non_object.intersection(
+                    mapper._server_default_plus_onupdate_propkeys)
+            )
+
+        if mapper.version_id_col is not None and \
                 mapper.version_id_generator is False:
             if mapper._version_id_prop.key in state.unloaded:
                 toload_now.extend([mapper._version_id_prop.key])