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

2 files changed, 9 insertions, 4 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index efe0ea2b4..5b7238a07 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -5674,8 +5674,6 @@ class DDLCompiler(Compiled):
 
         if prefix:
             text += prefix
-        if create.element.start is None:
-            create.element.start = self.dialect.default_sequence_base
         options = self.get_identity_options(create.element)
         if options:
             text += " " + options
diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index 5bfbd37c7..5d5f76c75 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -3323,7 +3323,7 @@ class Sequence(HasSchemaAttr, IdentityOptions, DefaultGenerator):
 
         some_table = Table(
             'some_table', metadata,
-            Column('id', Integer, Sequence('some_table_seq'),
+            Column('id', Integer, Sequence('some_table_seq', start=1),
             primary_key=True)
         )
 
@@ -3375,9 +3375,16 @@ class Sequence(HasSchemaAttr, IdentityOptions, DefaultGenerator):
 
         :param start: the starting index of the sequence.  This value is
          used when the CREATE SEQUENCE command is emitted to the database
-         as the value of the "START WITH" clause.   If ``None``, the
+         as the value of the "START WITH" clause. If ``None``, the
          clause is omitted, which on most platforms indicates a starting
          value of 1.
+
+         .. versionchanged:: 2.0 The :paramref:`.Sequence.start` parameter
+            is required in order to have DDL emit "START WITH".  This is a
+            reversal of a change made in version 1.4 which would implicitly
+            render "START WITH 1" if the :paramref:`.Sequence.start` were
+            not included.  See :ref:`change_7211` for more detail.
+
         :param increment: the increment value of the sequence.  This
          value is used when the CREATE SEQUENCE command is emitted to
          the database as the value of the "INCREMENT BY" clause.  If ``None``,