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, 8 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/requirements.py b/lib/sqlalchemy/testing/requirements.py
index af148a3b9..b001aaf75 100644
--- a/lib/sqlalchemy/testing/requirements.py
+++ b/lib/sqlalchemy/testing/requirements.py
@@ -288,6 +288,14 @@ class SuiteRequirements(Requirements):
         return exclusions.closed()
 
     @property
+    def server_side_cursors(self):
+        """Target dialect must support server side cursors."""
+
+        return exclusions.only_if([
+            lambda config: config.db.dialect.supports_server_side_cursors
+        ], "no server side cursors support")
+
+    @property
     def sequences(self):
         """Target database must support SEQUENCEs."""