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, 1 insertions, 0 deletions

diff --git a/lib/sqlalchemy/__init__.py b/lib/sqlalchemy/__init__.py
index 86c5afd8f..ad80a33c9 100644
--- a/lib/sqlalchemy/__init__.py
+++ b/lib/sqlalchemy/__init__.py
@@ -73,6 +73,7 @@ from .schema import ForeignKey as ForeignKey
 from .schema import ForeignKeyConstraint as ForeignKeyConstraint
 from .schema import Identity as Identity
 from .schema import Index as Index
+from .schema import insert_sentinel as insert_sentinel
 from .schema import MetaData as MetaData
 from .schema import PrimaryKeyConstraint as PrimaryKeyConstraint
 from .schema import Sequence as Sequence