Add support for server side cursors to mysqldb and pymysql

This allows to skip buffering of the results on the client side, e.g.
the following snippet:

    table = sa.Table(
        'testtbl', sa.MetaData(),
        sa.Column('id', sa.Integer, primary_key=True),
        sa.Column('a', sa.Integer),
        sa.Column('b', sa.String(512))
    )
    table.create(eng, checkfirst=True)

    with eng.connect() as conn:
        result = conn.execute(table.select().limit(1)).fetchone()
        if result is None:
            for _ in range(1000):
                conn.execute(
                    table.insert(),
                    [{'a': random.randint(1, 100000),
                      'b': ''.join(random.choice(string.ascii_letters) for _ in range(100))}
                      for _ in range(1000)]
                )

    with eng.connect() as conn:
        for row in conn.execution_options(stream_results=True).execute(table.select()):
            pass

now uses ~23 MB of memory instead of ~327 MB on CPython 3.5.2 and
PyMySQL 0.7.9.

psycopg2 implementation and execution options (stream_results,
server_side_cursors) are reused.

Change-Id: I4dc23ce3094f027bdff51b896b050361991c62e2

1 files changed, 14 insertions, 1 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/pymysql.py b/lib/sqlalchemy/dialects/mysql/pymysql.py
index 3c493fbfc..e29c17d8b 100644
--- a/lib/sqlalchemy/dialects/mysql/pymysql.py
+++ b/lib/sqlalchemy/dialects/mysql/pymysql.py
@@ -30,7 +30,7 @@ to the pymysql driver as well.
 """
 
 from .mysqldb import MySQLDialect_mysqldb
-from ...util import py3k
+from ...util import langhelpers, py3k
 
 
 class MySQLDialect_pymysql(MySQLDialect_mysqldb):
@@ -44,6 +44,19 @@ class MySQLDialect_pymysql(MySQLDialect_mysqldb):
     supports_unicode_statements = True
     supports_unicode_binds = True
 
+    def __init__(self, server_side_cursors=False, **kwargs):
+        super(MySQLDialect_pymysql, self).__init__(**kwargs)
+        self.server_side_cursors = server_side_cursors
+
+    @langhelpers.memoized_property
+    def supports_server_side_cursors(self):
+        try:
+            cursors = __import__('pymysql.cursors').cursors
+            self._sscursor = cursors.SSCursor
+            return True
+        except (ImportError, AttributeError):
+            return False
+
     @classmethod
     def dbapi(cls):
         return __import__('pymysql')