Revert automatic set of sequence start to 1

The :class:`.Sequence` construct restores itself to the DDL behavior it
had prior to the 1.4 series, where creating a :class:`.Sequence` with
no additional arguments will emit a simple ``CREATE SEQUENCE`` instruction
**without** any additional parameters for "start value".   For most backends,
this is how things worked previously in any case; **however**, for
MS SQL Server, the default value on this database is
``-2**63``; to prevent this generally impractical default
from taking effect on SQL Server, the :paramref:`.Sequence.start` parameter
should be provided.   As usage of :class:`.Sequence` is unusual
for SQL Server which for many years has standardized on ``IDENTITY``,
it is hoped that this change has minimal impact.

Fixes: #7211
Change-Id: I1207ea10c8cb1528a1519a0fb3581d9621c27b31

4 files changed, 43 insertions, 10 deletions

diff --git a/lib/sqlalchemy/dialects/mssql/base.py b/lib/sqlalchemy/dialects/mssql/base.py
index 085a2c27b..fdb0704f6 100644
--- a/lib/sqlalchemy/dialects/mssql/base.py
+++ b/lib/sqlalchemy/dialects/mssql/base.py
@@ -335,12 +335,31 @@ This is an auxiliary use case suitable for testing and bulk insert scenarios.
 SEQUENCE support
 ----------------
 
-The :class:`.Sequence` object now creates "real" sequences, i.e.,
-``CREATE SEQUENCE``. To provide compatibility with other dialects,
-:class:`.Sequence` defaults to a start value of 1, even though the
-T-SQL defaults is -9223372036854775808.
+The :class:`.Sequence` object creates "real" sequences, i.e.,
+``CREATE SEQUENCE``::
 
-.. versionadded:: 1.4.0
+    >>> from sqlalchemy import Sequence
+    >>> from sqlalchemy.schema import CreateSequence
+    >>> from sqlalchemy.dialects import mssql
+    >>> print(CreateSequence(Sequence("my_seq", start=1)).compile(dialect=mssql.dialect()))
+    CREATE SEQUENCE my_seq START WITH 1
+
+For integer primary key generation, SQL Server's ``IDENTITY`` construct should
+generally be preferred vs. sequence.
+
+..tip::
+
+    The default start value for T-SQL is ``-2**63`` instead of 1 as
+    in most other SQL databases. Users should explicitly set the
+    :paramref:`.Sequence.start` to 1 if that's the expected default::
+
+        seq = Sequence("my_sequence", start=1)
+
+.. versionadded:: 1.4 added SQL Server support for :class:`.Sequence`
+
+.. versionchanged:: 2.0 The SQL Server dialect will no longer implicitly
+   render "START WITH 1" for ``CREATE SEQUENCE``, which was the behavior
+   first implemented in version 1.4.
 
 MAX on VARCHAR / NVARCHAR
 -------------------------
@@ -2907,7 +2926,9 @@ class MSDialect(default.DefaultDialect):
 
     supports_sequences = True
     sequences_optional = True
-    # T-SQL's actual default is -9223372036854775808
+    # This is actually used for autoincrement, where itentity is used that
+    # starts with 1.
+    # for sequences T-SQL's actual default is -9223372036854775808
     default_sequence_base = 1
 
     supports_native_boolean = False
diff --git a/lib/sqlalchemy/dialects/mssql/provision.py b/lib/sqlalchemy/dialects/mssql/provision.py
index f307c7140..a7ecf4aa3 100644
--- a/lib/sqlalchemy/dialects/mssql/provision.py
+++ b/lib/sqlalchemy/dialects/mssql/provision.py
@@ -14,6 +14,7 @@ from ...testing.provision import drop_all_schema_objects_pre_tables
 from ...testing.provision import drop_db
 from ...testing.provision import get_temp_table_name
 from ...testing.provision import log
+from ...testing.provision import normalize_sequence
 from ...testing.provision import run_reap_dbs
 from ...testing.provision import temp_table_keyword_args
 
@@ -116,3 +117,10 @@ def drop_all_schema_objects_pre_tables(cfg, eng):
                             )
                         )
                     )
+
+
+@normalize_sequence.for_db("mssql")
+def normalize_sequence(cfg, sequence):
+    if sequence.start is None:
+        sequence.start = 1
+    return sequence
diff --git a/lib/sqlalchemy/dialects/oracle/base.py b/lib/sqlalchemy/dialects/oracle/base.py
index c06da6ffe..6481ae483 100644
--- a/lib/sqlalchemy/dialects/oracle/base.py
+++ b/lib/sqlalchemy/dialects/oracle/base.py
@@ -66,14 +66,14 @@ specify sequences, use the sqlalchemy.schema.Sequence object which is passed
 to a Column construct::
 
   t = Table('mytable', metadata,
-        Column('id', Integer, Sequence('id_seq'), primary_key=True),
+        Column('id', Integer, Sequence('id_seq', start=1), primary_key=True),
         Column(...), ...
   )
 
 This step is also required when using table reflection, i.e. autoload_with=engine::
 
   t = Table('mytable', metadata,
-        Column('id', Integer, Sequence('id_seq'), primary_key=True),
+        Column('id', Integer, Sequence('id_seq', start=1), primary_key=True),
         autoload_with=engine
   )
 
diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index f3db1a95e..5eab8f47c 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -27,9 +27,13 @@ default corresponding to the column.
 To specify a specific named sequence to be used for primary key generation,
 use the :func:`~sqlalchemy.schema.Sequence` construct::
 
-    Table('sometable', metadata,
-            Column('id', Integer, Sequence('some_id_seq'), primary_key=True)
+    Table(
+        "sometable",
+        metadata,
+        Column(
+            "id", Integer, Sequence("some_id_seq", start=1), primary_key=True
         )
+    )
 
 When SQLAlchemy issues a single INSERT statement, to fulfill the contract of
 having the "last insert identifier" available, a RETURNING clause is added to