Add sequence support for MariaDB 10.3+.

Added support for use of the :class:`.Sequence` construct with MariaDB 10.3
and greater, as this is now supported by this database.  The construct
integrates with the :class:`.Table` object in the same way that it does for
other databases like PostrgreSQL and Oracle; if is present on the integer
primary key "autoincrement" column, it is used to generate defaults.   For
backwards compatibility, to support a :class:`.Table` that has a
:class:`.Sequence` on it to support sequence only databases like Oracle,
while still not having the sequence fire off for MariaDB, the optional=True
flag should be set, which indicates the sequence should only be used to
generate the primary key if the target database offers no other option.

Fixes: #4976
Closes: #4996
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4996
Pull-request-sha: cb2e1426ea0b6bc6c93dbe8f033a11df9d8c4915

Change-Id: I507bc405eee6cae2c5991345d0eac53a37fe7512

1 files changed, 10 insertions, 2 deletions

diff --git a/lib/sqlalchemy/sql/crud.py b/lib/sqlalchemy/sql/crud.py
index 881ea9fcd..58abc10df 100644
--- a/lib/sqlalchemy/sql/crud.py
+++ b/lib/sqlalchemy/sql/crud.py
@@ -534,13 +534,21 @@ def _append_param_insert_pk(compiler, stmt, c, values, kw):
     no value passed in either; raise an exception.
 
     """
+
     if (
         # column has a Python-side default
         c.default is not None
         and (
-            # and it won't be a Sequence
+            # and it either is not a sequence, or it is and we support
+            # sequences and want to invoke it
             not c.default.is_sequence
-            or compiler.dialect.supports_sequences
+            or (
+                compiler.dialect.supports_sequences
+                and (
+                    not c.default.optional
+                    or not compiler.dialect.sequences_optional
+                )
+            )
         )
     ) or (
         # column is the "autoincrement column"