From d91095def47a9916300b6f1ce970befcd497381f Mon Sep 17 00:00:00 2001
From: Simon Charette <charette.s@gmail.com>
Date: Sun, 24 May 2020 09:18:12 -0400
Subject: [PATCH] [3.1.x] Fixed #31624 -- Avoided subquery usage on
 QuerySet.all().delete().

Thanks Adam Johnson for the report.

Regression in 7acef095d73322f45dcceb99afa1a4e50b520479.

Backport of 972000420e08703dd4981466ff67adcd5a61ad4b from master
---
 django/db/models/sql/compiler.py | 2 ++
 tests/delete/tests.py            | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/django/db/models/sql/compiler.py b/django/db/models/sql/compiler.py
index c368a59226..2c51561ae8 100644
--- a/django/db/models/sql/compiler.py
+++ b/django/db/models/sql/compiler.py
@@ -1400,6 +1400,8 @@ class SQLInsertCompiler(SQLCompiler):
 class SQLDeleteCompiler(SQLCompiler):
     @cached_property
     def single_alias(self):
+        # Ensure base table is in aliases.
+        self.query.get_initial_alias()
         return sum(self.query.alias_refcount[t] > 0 for t in self.query.alias_map) == 1
 
     def _as_sql(self, query):
diff --git a/tests/delete/tests.py b/tests/delete/tests.py
index c32ebba83f..e1ec26bc1a 100644
--- a/tests/delete/tests.py
+++ b/tests/delete/tests.py
@@ -605,6 +605,12 @@ class DeletionTests(TestCase):
 
 
 class FastDeleteTests(TestCase):
+    def test_fast_delete_all(self):
+        with self.assertNumQueries(1) as ctx:
+            User.objects.all().delete()
+        sql = ctx.captured_queries[0]['sql']
+        # No subqueries is used when performing a full delete.
+        self.assertNotIn('SELECT', sql)
 
     def test_fast_delete_fk(self):
         u = User.objects.create(