add deterministic imv returning ordering using sentinel columns

Repaired a major shortcoming which was identified in the
:ref:`engine_insertmanyvalues` performance optimization feature first
introduced in the 2.0 series. This was a continuation of the change in
2.0.9 which disabled the SQL Server version of the feature due to a
reliance in the ORM on apparent row ordering that is not guaranteed to take
place. The fix applies new logic to all "insertmanyvalues" operations,
which takes effect when a new parameter
:paramref:`_dml.Insert.returning.sort_by_parameter_order` on the
:meth:`_dml.Insert.returning` or :meth:`_dml.UpdateBase.return_defaults`
methods, that through a combination of alternate SQL forms, direct
correspondence of client side parameters, and in some cases downgrading to
running row-at-a-time, will apply sorting to each batch of returned rows
using correspondence to primary key or other unique values in each row
which can be correlated to the input data.

Performance impact is expected to be minimal as nearly all common primary
key scenarios are suitable for parameter-ordered batching to be
achieved for all backends other than SQLite, while "row-at-a-time"
mode operates with a bare minimum of Python overhead compared to the very
heavyweight approaches used in the 1.x series. For SQLite, there is no
difference in performance when "row-at-a-time" mode is used.

It's anticipated that with an efficient "row-at-a-time" INSERT with
RETURNING batching capability, the "insertmanyvalues" feature can be later
be more easily generalized to third party backends that include RETURNING
support but not necessarily easy ways to guarantee a correspondence
with parameter order.

Fixes: #9618
References: #9603
Change-Id: I1d79353f5f19638f752936ba1c35e4dc235a8b7c

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index 1ce5600e1..ad5e346b7 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -1468,6 +1468,7 @@ from ...sql import expression
 from ...sql import roles
 from ...sql import sqltypes
 from ...sql import util as sql_util
+from ...sql.compiler import InsertmanyvaluesSentinelOpts
 from ...sql.visitors import InternalTraversal
 from ...types import BIGINT
 from ...types import BOOLEAN
@@ -2911,6 +2912,12 @@ class PGDialect(default.DefaultDialect):
     postfetch_lastrowid = False
     use_insertmanyvalues = True
 
+    insertmanyvalues_implicit_sentinel = (
+        InsertmanyvaluesSentinelOpts.ANY_AUTOINCREMENT
+        | InsertmanyvaluesSentinelOpts.USE_INSERT_FROM_SELECT
+        | InsertmanyvaluesSentinelOpts.RENDER_SELECT_COL_CASTS
+    )
+
     supports_comments = True
     supports_constraint_comments = True
     supports_default_values = True