[1.11.x] Fixed #28062 -- Added a setting to disable server-side cursors on PostgreSQL.

When a connection pooler is set up in transaction pooling mode, queries
relying on server-side cursors fail. The DISABLE_SERVER_SIDE_CURSORS
setting in DATABASES disables server-side cursors for this use case.

Backport of 88336fdbb5e101fa25825b737169c0d6af2faa93 from master

3 files changed, 55 insertions, 0 deletions

diff --git a/docs/ref/databases.txt b/docs/ref/databases.txt
index 7a24ebbb0c..1a9f37afed 100644
--- a/docs/ref/databases.txt
+++ b/docs/ref/databases.txt
@@ -189,6 +189,41 @@ 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
 
+.. _transaction-pooling-server-side-cursors:
+
+Transaction pooling and server-side cursors
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 1.11.1
+
+Using a connection pooler in transaction pooling mode (e.g. `pgBouncer`_)
+requires disabling server-side cursors for that connection.
+
+Server-side cursors are local to a connection and remain open at the end of a
+transaction when :setting:`AUTOCOMMIT <DATABASE-AUTOCOMMIT>` is ``True``. A
+subsequent transaction may attempt to fetch more results from a server-side
+cursor. In transaction pooling mode, there's no guarantee that subsequent
+transactions will use the same connection. If a different connection is used,
+an error is raised when the transaction references the server-side cursor,
+because server-side cursors are only accessible in the connection in which they
+were created.
+
+One solution is to disable server-side cursors for a connection in
+:setting:`DATABASES` by setting :setting:`DISABLE_SERVER_SIDE_CURSORS
+<DATABASE-DISABLE_SERVER_SIDE_CURSORS>` to ``True``.
+
+To benefit from server-side cursors in transaction pooling mode, you could set
+up :doc:`another connection to the database </topics/db/multi-db>` in order to
+perform queries that use server-side cursors. This connection needs to either
+be directly to the database or to a connection pooler in session pooling mode.
+
+Another option is to wrap each ``QuerySet`` using server-side cursors in an
+:func:`~django.db.transaction.atomic` block, because it disables ``autocommit``
+for the duration of the transaction. This way, the server-side cursor will only
+live for the duration of the transaction.
+
+.. _pgBouncer: https://pgbouncer.github.io/
+
 Test database templates
 -----------------------
 
diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt
index 1d6a406cce..d49a2d27eb 100644
--- a/docs/ref/models/querysets.txt
+++ b/docs/ref/models/querysets.txt
@@ -2041,6 +2041,11 @@ 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.
 
+On PostgreSQL, server-side cursors will only be used when the
+:setting:`DISABLE_SERVER_SIDE_CURSORS <DATABASE-DISABLE_SERVER_SIDE_CURSORS>`
+setting is ``False``. Read :ref:`transaction-pooling-server-side-cursors` if
+you're using a connection pooler configured in transaction pooling mode.
+
 .. versionchanged:: 1.11
 
     PostgreSQL support for server-side cursors was added.
diff --git a/docs/ref/settings.txt b/docs/ref/settings.txt
index f585f8cf04..ceef7e8d98 100644
--- a/docs/ref/settings.txt
+++ b/docs/ref/settings.txt
@@ -646,6 +646,21 @@ PostgreSQL), it is an error to set this option.
 
 When :setting:`USE_TZ` is ``False``, it is an error to set this option.
 
+.. setting:: DATABASE-DISABLE_SERVER_SIDE_CURSORS
+
+``DISABLE_SERVER_SIDE_CURSORS``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 1.11.1
+
+Default: ``False``
+
+Set this to ``True`` if you want to disable the use of server-side cursors with
+:meth:`.QuerySet.iterator`. :ref:`transaction-pooling-server-side-cursors`
+describes the use case.
+
+This is a PostgreSQL-specific setting.
+
 .. setting:: USER
 
 ``USER``