Refs #16614 -- Made QuerySet.iterator() use server-side cursors on PostgreSQL.

Thanks to Josh Smeaton for the idea of implementing server-side cursors
in PostgreSQL from the iterator method, and Anssi Kääriäinen and Kevin
Turner for their previous work. Also Simon Charette and Tim Graham for
review.

django/db/backends/base/base.py

@@ -165,7 +165,7 @@ class BaseDatabaseWrapper(object):
         """Initializes the database connection settings."""
         raise NotImplementedError('subclasses of BaseDatabaseWrapper may require an init_connection_state() method')
 
-    def create_cursor(self):
+    def create_cursor(self, name=None):
         """Creates a cursor. Assumes that a connection is established."""
         raise NotImplementedError('subclasses of BaseDatabaseWrapper may require a create_cursor() method')
 
@@ -214,10 +214,21 @@ class BaseDatabaseWrapper(object):
 
     # ##### Backend-specific wrappers for PEP-249 connection methods #####
 
-    def _cursor(self):
+    def _prepare_cursor(self, cursor):
+        """
+        Validate the connection is usable and perform database cursor wrapping.
+        """
+        self.validate_thread_sharing()
+        if self.queries_logged:
+            wrapped_cursor = self.make_debug_cursor(cursor)
+        else:
+            wrapped_cursor = self.make_cursor(cursor)
+        return wrapped_cursor
+
+    def _cursor(self, name=None):
         self.ensure_connection()
         with self.wrap_database_errors:
-            return self.create_cursor()
+            return self.create_cursor(name)
 
     def _commit(self):
         if self.connection is not None:
@@ -240,12 +251,7 @@ class BaseDatabaseWrapper(object):
         """
         Creates a cursor, opening a connection if necessary.
         """
-        self.validate_thread_sharing()
-        if self.queries_logged:
-            cursor = self.make_debug_cursor(self._cursor())
-        else:
-            cursor = self.make_cursor(self._cursor())
-        return cursor
+        return self._prepare_cursor(self._cursor())
 
     def commit(self):
         """
@@ -553,6 +559,13 @@ class BaseDatabaseWrapper(object):
         """
         return DatabaseErrorWrapper(self)
 
+    def chunked_cursor(self):
+        """
+        Return a cursor that tries to avoid caching in the database (if
+        supported by the database), otherwise return a regular cursor.
+        """
+        return self.cursor()
+
     def make_debug_cursor(self, cursor):
         """
         Creates a cursor that logs all queries in self.queries_log.

django/db/backends/mysql/base.py

@@ -271,7 +271,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
                 # with SQL standards.
                 cursor.execute('SET SQL_AUTO_IS_NULL = 0')
 
-    def create_cursor(self):
+    def create_cursor(self, name=None):
         cursor = self.connection.cursor()
         return CursorWrapper(cursor)
 

django/db/backends/oracle/base.py

@@ -249,7 +249,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         if not self.get_autocommit():
             self.commit()
 
-    def create_cursor(self):
+    def create_cursor(self, name=None):
         return FormatStylePlaceholderCursor(self.connection)
 
     def _commit(self):

django/db/backends/postgresql/base.py

@@ -4,6 +4,7 @@ PostgreSQL database backend for Django.
 Requires psycopg 2: http://initd.org/projects/psycopg2
 """
 
+import threading
 import warnings
 
 from django.conf import settings
@@ -145,6 +146,10 @@ class DatabaseWrapper(BaseDatabaseWrapper):
     introspection_class = DatabaseIntrospection
     ops_class = DatabaseOperations
 
+    def __init__(self, *args, **kwargs):
+        super(DatabaseWrapper, self).__init__(*args, **kwargs)
+        self._named_cursor_idx = 0
+
     def get_connection_params(self):
         settings_dict = self.settings_dict
         # None may be used to connect to the default 'postgres' db
@@ -206,11 +211,27 @@ class DatabaseWrapper(BaseDatabaseWrapper):
             if not self.get_autocommit():
                 self.connection.commit()
 
-    def create_cursor(self):
-        cursor = self.connection.cursor()
+    def create_cursor(self, name=None):
+        if name:
+            # In autocommit mode, the cursor will be used outside of a
+            # transaction, hence use a holdable cursor.
+            cursor = self.connection.cursor(name, scrollable=False, withhold=self.connection.autocommit)
+        else:
+            cursor = self.connection.cursor()
         cursor.tzinfo_factory = utc_tzinfo_factory if settings.USE_TZ else None
         return cursor
 
+    def chunked_cursor(self):
+        self._named_cursor_idx += 1
+        db_cursor = self._cursor(
+            name='_django_curs_%d_%d' % (
+                # Avoid reusing name in other threads
+                threading.current_thread().ident,
+                self._named_cursor_idx,
+            )
+        )
+        return self._prepare_cursor(db_cursor)
+
     def _set_autocommit(self, autocommit):
         with self.wrap_database_errors:
             self.connection.autocommit = autocommit

django/db/backends/sqlite3/base.py

@@ -215,7 +215,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
     def init_connection_state(self):
         pass
 
-    def create_cursor(self):
+    def create_cursor(self, name=None):
         return self.connection.cursor(factory=SQLiteCursorWrapper)
 
     def close(self):

django/db/models/query.py

@@ -34,8 +34,9 @@ EmptyResultSet = sql.EmptyResultSet
 
 
 class BaseIterable(object):
-    def __init__(self, queryset):
+    def __init__(self, queryset, chunked_fetch=False):
         self.queryset = queryset
+        self.chunked_fetch = chunked_fetch
 
 
 class ModelIterable(BaseIterable):
@@ -49,7 +50,7 @@ class ModelIterable(BaseIterable):
         compiler = queryset.query.get_compiler(using=db)
         # Execute the query. This will also fill compiler.select, klass_info,
         # and annotations.
-        results = compiler.execute_sql()
+        results = compiler.execute_sql(chunked_fetch=self.chunked_fetch)
         select, klass_info, annotation_col_map = (compiler.select, compiler.klass_info,
                                                   compiler.annotation_col_map)
         model_cls = klass_info['model']
@@ -318,7 +319,7 @@ class QuerySet(object):
         An iterator over the results from applying this QuerySet to the
         database.
         """
-        return iter(self._iterable_class(self))
+        return iter(self._iterable_class(self, chunked_fetch=True))
 
     def aggregate(self, *args, **kwargs):
         """
@@ -1071,7 +1072,7 @@ class QuerySet(object):
 
     def _fetch_all(self):
         if self._result_cache is None:
-            self._result_cache = list(self.iterator())
+            self._result_cache = list(self._iterable_class(self))
         if self._prefetch_related_lookups and not self._prefetch_done:
             self._prefetch_related_objects()
 

django/db/models/sql/compiler.py

@@ -799,7 +799,7 @@ class SQLCompiler(object):
         self.query.set_extra_mask(['a'])
         return bool(self.execute_sql(SINGLE))
 
-    def execute_sql(self, result_type=MULTI):
+    def execute_sql(self, result_type=MULTI, chunked_fetch=False):
         """
         Run the query against the database and returns the result(s). The
         return value is a single data item if result_type is SINGLE, or an
@@ -823,12 +823,16 @@ class SQLCompiler(object):
                 return iter([])
             else:
                 return
-
-        cursor = self.connection.cursor()
+        if chunked_fetch:
+            cursor = self.connection.chunked_cursor()
+        else:
+            cursor = self.connection.cursor()
         try:
             cursor.execute(sql, params)
         except Exception:
-            cursor.close()
+            with self.connection.wrap_database_errors:
+                # Closing a server-side cursor could yield an error
+                cursor.close()
             raise
 
         if result_type == CURSOR:
@@ -852,11 +856,11 @@ class SQLCompiler(object):
             cursor, self.connection.features.empty_fetchmany_value,
             self.col_count
         )
-        if not self.connection.features.can_use_chunked_reads:
+        if not chunked_fetch and not self.connection.features.can_use_chunked_reads:
             try:
                 # If we are using non-chunked reads, we return the same data
                 # structure as normally, but ensure it is all read into memory
-                # before going any further.
+                # before going any further. Use chunked_fetch if requested.
                 return list(result)
             finally:
                 # done with the cursor

django/test/testcases.py

@@ -175,6 +175,7 @@ class SimpleTestCase(unittest.TestCase):
             for alias in connections:
                 connection = connections[alias]
                 connection.cursor = _CursorFailure(cls.__name__, connection.cursor)
+                connection.chunked_cursor = _CursorFailure(cls.__name__, connection.chunked_cursor)
 
     @classmethod
     def tearDownClass(cls):
@@ -182,6 +183,7 @@ class SimpleTestCase(unittest.TestCase):
             for alias in connections:
                 connection = connections[alias]
                 connection.cursor = connection.cursor.wrapped
+                connection.chunked_cursor = connection.chunked_cursor.wrapped
         if hasattr(cls, '_cls_modified_context'):
             cls._cls_modified_context.disable()
             delattr(cls, '_cls_modified_context')

docs/ref/databases.txt

@@ -171,6 +171,24 @@ If you need to add a PostgreSQL extension (like ``hstore``, ``postgis``, etc.)
 using a migration, use the
 :class:`~django.contrib.postgres.operations.CreateExtension` operation.
 
+.. _postgresql-server-side-cursors:
+
+Server-side cursors
+-------------------
+
+.. versionadded:: 1.11
+
+When using :meth:`QuerySet.iterator()
+<django.db.models.query.QuerySet.iterator>`, Django opens a :ref:`server-side
+cursor <psycopg2:server-side-cursors>`. By default, PostgreSQL assumes that
+only the first 10% of the results of cursor queries will be fetched. The query
+planner spends less time planning the query and starts returning results
+faster, but this could diminish performance if more than 10% of the results are
+retrieved. PostgreSQL's assumptions on the number of rows retrieved for a
+cursor query is controlled with the `cursor_tuple_fraction`_ option.
+
+.. _cursor_tuple_fraction: https://www.postgresql.org/docs/current/static/runtime-config-query.html#GUC-CURSOR-TUPLE-FRACTION
+
 Test database templates
 -----------------------
 

docs/ref/models/querysets.txt

@@ -1981,15 +1981,15 @@ evaluated will force it to evaluate again, repeating the query.
 Also, use of ``iterator()`` causes previous ``prefetch_related()`` calls to be
 ignored since these two optimizations do not make sense together.
 
-.. warning::
+Some Python database drivers still load the entire result set into memory, but
+won't cache results after iterating over them. Oracle and :ref:`PostgreSQL
+<postgresql-server-side-cursors>` use server-side cursors to stream results
+from the database without loading the entire result set into memory.
+
+.. versionchanged:: 1.11
 
-    Some Python database drivers like ``psycopg2`` perform caching if using
-    client side cursors (instantiated with ``connection.cursor()`` and what
-    Django's ORM uses). Using ``iterator()`` does not affect caching at the
-    database driver level. To disable this caching, look at `server side
-    cursors`_.
+    PostgreSQL support for server-side cursors was added.
 
-.. _server side cursors: http://initd.org/psycopg/docs/usage.html#server-side-cursors
 
 ``latest()``
 ~~~~~~~~~~~~

docs/releases/1.11.txt

@@ -273,6 +273,11 @@ Database backends
 * Added the :setting:`TEST['TEMPLATE'] <TEST_TEMPLATE>` setting to let
   PostgreSQL users specify a template for creating the test database.
 
+* :meth:`.QuerySet.iterator()` now uses :ref:`server-side cursors
+  <psycopg2:server-side-cursors>` on PostgreSQL. This feature transfers some of
+  the worker memory load (used to hold query results) to the database and might
+  increase database memory usage.
+
 Email
 ~~~~~
 
@@ -527,6 +532,10 @@ Database backend API
 * Renamed the ``ignores_quoted_identifier_case`` feature to
   ``ignores_table_name_case`` to more accurately reflect how it is used.
 
+* The ``name`` keyword argument is added to the
+  ``DatabaseWrapper.create_cursor(self, name=None)`` method to allow usage of
+  server-side cursors on backends that support it.
+
 Dropped support for PostgreSQL 9.2 and PostGIS 2.0
 --------------------------------------------------
 

tests/backends/test_postgresql.py

@@ -0,0 +1,54 @@
+import unittest
+from collections import namedtuple
+
+from django.db import connection
+from django.test import TestCase
+
+from .models import Person
+
+
+@unittest.skipUnless(connection.vendor == 'postgresql', "Test only for PostgreSQL")
+class ServerSideCursorsPostgres(TestCase):
+    cursor_fields = 'name, statement, is_holdable, is_binary, is_scrollable, creation_time'
+    PostgresCursor = namedtuple('PostgresCursor', cursor_fields)
+
+    @classmethod
+    def setUpTestData(cls):
+        Person.objects.create(first_name='a', last_name='a')
+        Person.objects.create(first_name='b', last_name='b')
+
+    def inspect_cursors(self):
+        with connection.cursor() as cursor:
+            cursor.execute('SELECT {fields} FROM pg_cursors;'.format(fields=self.cursor_fields))
+            cursors = cursor.fetchall()
+        return [self.PostgresCursor._make(cursor) for cursor in cursors]
+
+    def test_server_side_cursor(self):
+        persons = Person.objects.iterator()
+        next(persons)  # Open a server-side cursor
+        cursors = self.inspect_cursors()
+        self.assertEqual(len(cursors), 1)
+        self.assertIn('_django_curs_', cursors[0].name)
+        self.assertFalse(cursors[0].is_scrollable)
+        self.assertFalse(cursors[0].is_holdable)
+        self.assertFalse(cursors[0].is_binary)
+
+    def test_server_side_cursor_many_cursors(self):
+        persons = Person.objects.iterator()
+        persons2 = Person.objects.iterator()
+        next(persons)  # Open a server-side cursor
+        next(persons2)  # Open a second server-side cursor
+        cursors = self.inspect_cursors()
+        self.assertEqual(len(cursors), 2)
+        for cursor in cursors:
+            self.assertIn('_django_curs_', cursor.name)
+            self.assertFalse(cursor.is_scrollable)
+            self.assertFalse(cursor.is_holdable)
+            self.assertFalse(cursor.is_binary)
+
+    def test_closed_server_side_cursor(self):
+        persons = Person.objects.iterator()
+        next(persons)  # Open a server-side cursor
+        del persons
+        cursors = self.inspect_cursors()
+        self.assertEqual(len(cursors), 0)

tests/test_utils/tests.py

@@ -1069,6 +1069,18 @@ class DisallowedDatabaseQueriesTests(SimpleTestCase):
             Car.objects.first()
 
 
+class DisallowedDatabaseQueriesChunkedCursorsTests(SimpleTestCase):
+    def test_disallowed_database_queries(self):
+        expected_message = (
+            "Database queries aren't allowed in SimpleTestCase. Either use "
+            "TestCase or TransactionTestCase to ensure proper test isolation or "
+            "set DisallowedDatabaseQueriesChunkedCursorsTests.allow_database_queries "
+            "to True to silence this failure."
+        )
+        with self.assertRaisesMessage(AssertionError, expected_message):
+            next(Car.objects.iterator())
+
+
 class AllowedDatabaseQueriesTests(SimpleTestCase):
     allow_database_queries = True