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

2 files changed, 57 insertions, 29 deletions

diff --git a/lib/sqlalchemy/dialects/mssql/base.py b/lib/sqlalchemy/dialects/mssql/base.py
index f32665792..4a7e48ab8 100644
--- a/lib/sqlalchemy/dialects/mssql/base.py
+++ b/lib/sqlalchemy/dialects/mssql/base.py
@@ -251,12 +251,16 @@ The process for fetching this value has several variants:
 
     INSERT INTO t (x) OUTPUT inserted.id VALUES (?)
 
-  .. note::  SQLAlchemy 2.0 introduced the :ref:`engine_insertmanyvalues`
-     feature for SQL Server, which is used by default to optimize many-row
-     INSERT statements; however as of SQLAlchemy 2.0.9 this feature is
-     temporarily disabled for SQL Server, until adjustments can be made
-     so that the ORM unit of work does not rely upon the ordering of returned
-     rows.
+  As of SQLAlchemy 2.0, the :ref:`engine_insertmanyvalues` feature is also
+  used by default to optimize many-row INSERT statements; for SQL Server
+  the feature takes place for both RETURNING and-non RETURNING
+  INSERT statements.
+
+  .. versionchanged:: 2.0.10 The :ref:`engine_insertmanyvalues` feature for
+     SQL Server was temporarily disabled for SQLAlchemy version 2.0.9 due to
+     issues with row ordering. As of 2.0.10 the feature is re-enabled, with
+     special case handling for the unit of work's requirement for RETURNING to
+     be ordered.
 
 * When RETURNING is not available or has been disabled via
   ``implicit_returning=False``, either the ``scope_identity()`` function or
@@ -936,6 +940,7 @@ from ...sql import roles
 from ...sql import sqltypes
 from ...sql import util as sql_util
 from ...sql._typing import is_sql_compiler
+from ...sql.compiler import InsertmanyvaluesSentinelOpts
 from ...types import BIGINT
 from ...types import BINARY
 from ...types import CHAR
@@ -1514,6 +1519,39 @@ class MSUUid(sqltypes.Uuid):
 
                 return process
 
+    def _sentinel_value_resolver(self, dialect):
+        """Return a callable that will receive the uuid object or string
+        as it is normally passed to the DB in the parameter set, after
+        bind_processor() is called.  Convert this value to match
+        what it would be as coming back from an INSERT..OUTPUT inserted.
+
+        for the UUID type, there are four varieties of settings so here
+        we seek to convert to the string or UUID representation that comes
+        back from the driver.
+
+        """
+        character_based_uuid = (
+            not dialect.supports_native_uuid or not self.native_uuid
+        )
+
+        if character_based_uuid:
+            if self.native_uuid:
+                # for pyodbc, uuid.uuid() objects are accepted for incoming
+                # data, as well as strings. but the driver will always return
+                # uppercase strings in result sets.
+                def process(value):
+                    return str(value).upper()
+
+            else:
+
+                def process(value):
+                    return str(value)
+
+            return process
+        else:
+            # for pymssql, we get uuid.uuid() objects back.
+            return None
+
 
 class UNIQUEIDENTIFIER(sqltypes.Uuid[sqltypes._UUID_RETURN]):
     __visit_name__ = "UNIQUEIDENTIFIER"
@@ -2995,13 +3033,18 @@ class MSDialect(default.DefaultDialect):
     # may be changed at server inspection time for older SQL server versions
     supports_multivalues_insert = True
 
-    # disabled due to #9603
-    use_insertmanyvalues = False
+    use_insertmanyvalues = True
 
     # note pyodbc will set this to False if fast_executemany is set,
     # as of SQLAlchemy 2.0.9
     use_insertmanyvalues_wo_returning = True
 
+    insertmanyvalues_implicit_sentinel = (
+        InsertmanyvaluesSentinelOpts.AUTOINCREMENT
+        | InsertmanyvaluesSentinelOpts.IDENTITY
+        | InsertmanyvaluesSentinelOpts.USE_INSERT_FROM_SELECT
+    )
+
     # "The incoming request has too many parameters. The server supports a "
     # "maximum of 2100 parameters."
     # in fact you can have 2099 parameters.
@@ -3064,14 +3107,6 @@ class MSDialect(default.DefaultDialect):
 
         super().__init__(**opts)
 
-        if self.use_insertmanyvalues:
-            raise exc.ArgumentError(
-                "The use_insertmanyvalues feature on SQL Server is currently "
-                "not safe to use, as returned result rows may be returned in "
-                "random order.  Ensure use_insertmanyvalues is left at its "
-                "default of False (this setting changed in SQLAlchemy 2.0.9)"
-            )
-
         self._json_serializer = json_serializer
         self._json_deserializer = json_deserializer
 
diff --git a/lib/sqlalchemy/dialects/mssql/pyodbc.py b/lib/sqlalchemy/dialects/mssql/pyodbc.py
index 08c6bc48f..6af527e73 100644
--- a/lib/sqlalchemy/dialects/mssql/pyodbc.py
+++ b/lib/sqlalchemy/dialects/mssql/pyodbc.py
@@ -290,19 +290,6 @@ Pyodbc have been resolved as of SQLAlchemy 2.0.5. See the notes at
 Fast Executemany Mode
 ---------------------
 
-  .. note::  SQLAlchemy 2.0 introduced the :ref:`engine_insertmanyvalues`
-     feature for SQL Server, which is used by default to optimize many-row
-     INSERT statements; however as of SQLAlchemy 2.0.9 this feature had
-     to be turned off for SQL Server as the database does not support
-     deterministic RETURNING of INSERT rows for a multi-row INSERT statement.
-
-.. versionchanged:: 2.0.9 - ``fast_executemany`` executions will be used
-   for INSERT statements that don't include RETURNING, when
-   ``fast_executemany`` is set. Previously, ``use_insertmanyvalues`` would
-   cause ``fast_executemany`` to not be used in most cases.
-
-   ``use_insertmanyvalues`` is disabled for SQL Server overall as of 2.0.9.
-
 The PyODBC driver includes support for a "fast executemany" mode of execution
 which greatly reduces round trips for a DBAPI ``executemany()`` call when using
 Microsoft ODBC drivers, for **limited size batches that fit in memory**.  The
@@ -316,6 +303,12 @@ Server dialect supports this parameter by passing the
         "mssql+pyodbc://scott:tiger@mssql2017:1433/test?driver=ODBC+Driver+17+for+SQL+Server",
         fast_executemany=True)
 
+.. versionchanged:: 2.0.9 - the ``fast_executemany`` parameter now has its
+   intended effect of this PyODBC feature taking effect for all INSERT
+   statements that are executed with multiple parameter sets, which don't
+   include RETURNING.  Previously, SQLAlchemy 2.0's :term:`insertmanyvalues`
+   feature would cause ``fast_executemany`` to not be used in most cases
+   even if specified.
 
 .. versionadded:: 1.3